Shelby Sheffield has no good explanation for what happened last year. Suddenly she found herself a world-class competitor in what many consider to be the most grueling athletic endurance event on the planet: the triathlon.

On the 27th floor of the City Center downtown, in a conference room at the high-powered law firm of Waller Lansden, where she is an attorney, she picks through the possible reasons. A petite woman in a long-sleeved black, button-up sweater and a calf-length skirt, she is speaking so softly and self-deprecatingly that you wonder how on earth such a nice person could compete in such an arena, and at such a high level. The reasons for the improvement begin pouring out.

First is that the years of agonizing workouts—rolling out of bed most weekdays at 4:30 a.m. to run, followed by a swim routine, not to mention the back-to-back bike rides and runs on the weekends—finally just added up. Rather than improve bit by bit at a time, she surmises, she just improved all at once. There’s the issue of her weight: When she decided to quit worrying about it and started eating things she had never eaten before (peanut butter, for instance), 15 pounds disappeared. That helped her runs in particular. A third and final reason, she says, is that she changed the medicine she had been taking for migraine headaches, and her heart rate sped up, allowing her to train farther, harder, faster.

“Last year the sky just opened up. I just don’t know why,” she shrugs.

In 2002, the 29-year-old Sheffield went from being a much respected triathlete on a regional level to placing second in her age group at the USA Triathlon National Championships. Her times were astonishing: In Nashville, Louisville and Decatur, Ala., she set course records. In fact, she has amassed enough top showings to enable her to go professional—a choice she has yet to make. In what was probably her crowning achievement, in April of this year, she qualified to compete in the Hawaii Ironman Triathlon, the most prestigious triathlon in the world.

What may be most remarkable about Sheffield is that she is still improving by leaps and bounds. Last year, owing primarily to her improvement as a runner, her times just kept falling and falling. Those who follow the sport of triathlon know Sheffield is one of the best. The question is how much further she can go.

Triathletes of Sheffield’s quality operate at the extremes of human endurance. Their willingness to subject themselves to intense pain and discomfort, and over long periods of time, is mind-numbing. Competing and training for triathlons is not something that one does every once in a while. It is something that occupies a life.

Triathlons consist of three basic elements—a swim, a bike ride and a run—and are of varying lengths. So-called “Ironman” triathlons have a 2.4-mile swim, a 112-mile bike ride and a 26.2-mile (marathon-length) run. (Combined, that’s longer than the distance from Nashville to Chattanooga.) There are half-Ironman races and there are “Olympic” distance triathlons, which are one-fourth as long. All are punishing.

The swim, almost always held in a lake or ocean, has the effect of exhausting the upper body. Participants are frequently kicked, punched or dunked by fellow swimmers, and the water can be choppy and cold. The bike ride, which is the longest single time element in a triathlon, takes its toll in several ways. Because it goes on for so long, the carbohydrates, glycogen and electrolytes burn off, depleting the body of energy stores. As well, a bike rider’s thighs are pounded so relentlessly that, by the ride’s end, the quad muscles can barely twitch. Unfortunately, that’s when the run starts. Just about all triathletes will tell you that when the run commences, it’s all they can do to point their bodies forward and hope for a recovery somewhere down the road. Sometimes it never comes.

While Sheffield can’t point to one single explanation for her breakthrough year in 2002, she was aware enough to know the moment when it arrived. Her coach, Robert Eslick, had sensed that something was up well before she did.

“There’s a certain test I have her do every month—it’s a maximum aerobic fitness test—in which she does interval workouts and I measure her heart rate,” he says. “What was happening was she was getting faster, but her heart rate was remaining the same. That’s a good indication of aerobic fitness, and it’s also an indication that she’s not overtraining or under stress.” Eslick, who has been working with Sheffield ever since he gave her a workout schedule for her first marathon nine years ago, says his suspicions about her improvement were confirmed only a few weeks later. “She ran a 5K race last July 4, and it was the first time she broke 20 minutes. Basically, I told her she had to start relaxing. Because I knew she was there.”

Sheffield remembers the tests, and she remembers the 5K time, and while that was all fine and good, she says, “None of that made me think I was necessarily going to jump through any mental hurdles with my triathlons.” But that all changed at the 2002 USA Triathlon National Championships later that summer. The “nationals,” which were held last year in Coeur d’Alene, Idaho, determine the top amateur triathletes in the country. Winners of the event then participate in the world amateur championships, which are to be held this year in Queenstown, New Zealand. The world championship is Olympic distance: The swim is 1.5K (just under 1 mile); the bike is 40K (about 25 miles); the run is 10K (6.2 miles).

Sheffield had been debating whether she should even go to the nationals in Idaho, until one day she found herself on a bike ride with Bruce Gennari, a well-established Nashville triathlete who competes on the national circuit. “He told me I needed to go and just see how I stack up. He told me I’d be sorry if I didn’t. Now, Bruce has inspired me for years. And for someone like him to say I was good was so special it makes me cry,” she says, her eyes filling with tears.

Once in Idaho, Sheffield says she found herself in paradise, both because of the beauty of the place and how she felt about her conditioning. “There was no pressure on me. I had done the training, and I’d tapered and I had nothing to lose. Bruce had convinced me I had what it took to go to New Zealand. I was just totally ready.”

As Sheffield stood in a crystal-clear lake with about 70 other women in her 25- to 29-year-old age group, preparing to begin the swim, an announcer described the hot-shot athletes in Sheffield’s wave. He singled out a triathlete from California and described the particular races she had won, then discussed an East Coast participant and her recent victories. “Of course, they passed over me,” Sheffield says.

When the gun went off, Sheffield surged to the front. “I almost always do well in the swim,” she says in a low voice that makes it clear just how much she hates talking about how good she is. “I passed a lot of people.” The next event also went well. “I had a strong bike. It was a really hilly course, and I like to attack hills. I passed a lot of people then.”

Then came the run. “The first mile is always the hardest. It’s brutal. My mantra is, 'It’s always going to get better.’ I have to remind myself when I start that I will start feeling better in about 20 minutes. You can’t believe it won’t get better or you’re sunk. You just have to believe.”

As the run went on, and she got her legs, she began playing, again and again, a song in her head: “Just Like a Pill,” by the spiky-haired pop star P!nk. She and several other Nashville women had been dancing to it in their hotel room just before the race. “I mean, we were really bouncing around.” One line in the song particularly pumped her up: “Run just as fast as I can, to the middle of nowhere, to the middle of my frustrated fears.”

And run she did. That day she recorded the fastest 10K she had ever run. As she crossed the finish line, clocking a time of two hours and 16 minutes, Gennari and his wife were waiting for Sheffield, grins spreading across their faces. She had placed second in her age group, and fifth overall out of all the women, which meant she had qualified for the world championships in New Zealand. She had come out of nowhere.

“She’s no longer a no-name,” says Gennari, a former swimmer from the University of Alabama who has a god-like reputation in Tennessee triathlon circles. “She’s now in the upper echelons of the amateur scene in the country, and I know she has what it takes to beat some of the pros.”

Of course, there is one slight problem. “There’s only one person who may not know she has the stuff, and that would be her,” he adds. I’m always having to go up to her and say, 'Listen, kiddo, you’re good. You just have to come to grips with that.’ ”

Getting Sheffield to talk about how good she is can be downright frustrating. “If you asked me what my strengths are, I don’t know if I could tell you that,” she says. When she talks, in fact, she spends much of her time saying how bad she is. “I really don’t know how to ride a bike well, and so I just pedal hard.... I worry about crashing. I’ve got a lot to learn.” Or with the swim: “My eyes aren’t real good, and sometimes I can’t see the buoys [that mark the course]. If you see a person swimming off-course, that’s me.”

At Waller Lansden, where she has been working for two years in regulatory law, her supervisor, James Weaver, relates a story that just about sums up her inordinate reserves of humility.

“When we hired her, her résumé had said she does triathlons. Well, lots of people do triathlons these days,” Weaver says. “One day someone came into my office and said, 'Do you know about Shelby?’ And I said, 'Yeah, I know about her, she works for me and I hear she likes to run.’ And they said, 'No, do you really know—that she’s competing on an international level?,’ and they showed me a Web site. And it listed all this amazing stuff about her. So I went right over to her and I said, 'How could you not tell me about this?’ You literally have to beat her with a stick to get her to say she finished first.”

Eslick, Sheffield’s coach, says her humility can nevertheless be deceiving. “If you met her in a social situation, and didn’t know who she was, you’d never know she was as competitive as she is. But when she gets in a race, a completely different face goes on.”

As with many successful triathletes, Sheffield’s origins in the sport can be traced back to her time spent in the swimming pool. Growing up in Louisville, she swam competitively, doing well enough to swim for Transylvania University in Lexington. It was around this time she began to exhibit some skill at endurance, or being able to withstand physical exertion for long periods of time. One story from her senior year provides a shocking glimpse into the level at which she can train.

At a workout in Florida, where her college team was practicing between the fall and spring semesters, a coach approached her to say that he had devised a tough workout for her that day. He said he would not ask her to do anything he did not feel she could do.

The workout called for swimming 10,000 meters—or 6.2 miles—of butterfly, the most exhausting stroke there is. There would be no breaks. She would swim it straight through. “I had confidence in my coach, so I knew I could do it,” she says.

Swimming butterfly requires not just more technique than all the other strokes, but more strength as well. Swimming 200 yards of it is impressive. Completing a mile of it is extremely rare. Doing 10K of butterfly is insane. But two hours and 38 minutes later, with the entire team spread out on both sides of the pool cheering her on, she finished. “It didn’t really hurt that much because my body was pretty much numb,” she says. “It was actually more of a mental thing. As it turned out, every other thing I came across that year in the pool was a joke because I had already done something as difficult as that.”

Her senior year at college, she also ran the Marine Corps Marathon, signing up as part of the Leukemia “Team in Training” program for beginning marathoners. That connected her with Eslick, who coached her through the event. She remembers her time as being nearly five hours, which is decidedly slow. “Oprah beat me,” she recalls.

Graduating from Transylvania in 1995, Sheffield headed to Nashville for the summer and never left. She was introduced to her future husband, Bob, a Metro cop. One day she borrowed her brother’s bike, which was of the kickstand variety, and took part in a huge triathlon in Chicago. “I’m riding this catastrophe of a bike, but I’m definitely an endurance person, so it didn’t really take it out of me,” she says.

She worked odd jobs in Nashville, ultimately becoming a paralegal. Then she enrolled in the University of Memphis Law School, from which she graduated in 2001. She continued doing all of Eslick’s workouts—which he has been sending her every two weeks for nearly a decade now—and she began doing more and more triathlons (though she did swear off of them temporarily, when law school began to take its toll).

In 2001, Sheffield went to work for Waller Lansden, and she got involved with the Greater Nashville Athletic Club (GNAC), which promotes outdoor activities and is big into triathlons. “I was just your basic recreational triathlete,” she says. But she was also getting very good, quite often finishing in the top three in her age division. She was a frequent standout at Nashville’s Music City Triathlon, a tricky—and taxing—event held in early September. Meanwhile, a considerable number of her workout friends began coaxing her to think more ambitiously about what she could accomplish.

Forty-three-year-old Kathleen Johnston is a committed triathlete who has been ranked in the top 10 amateurs in the country and knows Sheffield well. The two have run many a mile together. “I think once she got law school out of the way, and she found a job, then things began to take off. It usually takes several years to get to a national level, and a lot of that is mental, getting to a point where you believe you can beat people. All of it just feeds on itself.”

When Johnston started noticing Sheffield’s performances last year, she says she saw that all the pieces were falling in place. “It just escalated, exponentially, up and up and up.”

As the 2002 successes began rolling in, and with her friends encouraging her to think big, Sheffield herself says that one event began to dominate her thinking: the Hawaii Ironman. And so she set her sights on the triathlon’s ultimate challenge.

The first triathlon ever held was the Ironman-distance event in Kona, Hawaii, in 1979. The winning time that year was 11 hours, 46 minutes. As lore has it, the event was devised by a Navy commander as a way to settle an argument about who was the most fit: swimmers, runners or bikers. Since that time, other Ironman-distance triathlons have sprung up around the world, but the Hawaii event is considered the granddaddy of them all. Winners in recent years have crossed the finish line in somewhere between eight and nine hours.

Sheffield says midway through last year, she began thinking, “You know, I’m good in the swim, and my bike has come along way, and if I can get a run together, I could qualify for Hawaii. It was like a light bulb had gone off.”

While the Hawaii event is difficult enough to complete, it’s even harder to qualify for, because it’s considered the Holy Grail of the sport. Since its founding 25 years ago, it has drawn every seriously competitive triathlete from around the globe. The entry slots, which are determined in various qualifying races held around the world, are few and far between.

Sheffield looked at the qualifying races for the 2003 Hawaii race and decided she would compete in three. There was a half-Ironman distance triathlon in California this April, another half in St. Croix in May, and a full Ironman distance in New York in July. So this past New Year’s Day, her training went into overdrive.

“I live near David Lipscomb, and I would get on my bike at my house and go through Percy Warner Park, and then hit the Natchez Trace Parkway, and along the way I would try to climb every hill I could find in between. I would just torture myself.” When the snow fell, and she couldn’t work out outdoors, she would sit on a stationary bike in her house for three hours at a time, gutting it out. “I would make my husband come sit down and talk to me while I pedaled,” she says. “Or I would watch a whole Titans game. It just gets boring.”

The event in April was the Ralph’s Half Ironman California, held in the town of Oceanside. To qualify for Hawaii meant she would have to win her age group. “The thing that worried me about the race is I would be competing against all these other Californians who would have had a chance to train in good weather. Meanwhile, I was going to have trained in a Nashville winter.”

Sheffield once again did phenomenally well in the 1.2-mile swim. In the 56-mile bike, she excelled. But pulling into the transition area to go from the bike to the run, another runner in her age bracket caught up to her. While putting on her running shoes, Sheffield said, “I decided to just race it smart.” When the two took off for the half-marathon run, both of them apparently leading the pack, Shelby says, “I just got behind her and trailed her.”

At mile eight, Sheffield says she pulled up beside the woman. “Since she had caught me on the bike, I congratulated her on that. She then told me I was running a good race myself. I asked her what her name was, and she told me. And then, out of nowhere, she said, 'Just so you know, if I get the Hawaii slot I’m not taking it.’ She also said, 'At the rate you’re going, I don’t think you have anything to worry about, but I thought you’d like to know.’ ”

Suddenly, Sheffield’s dream of participating in the world’s greatest triathlon was not only within reach, it seemed like it was hers for the asking. All she had to do was finish the race. Something else happened entirely: “At that point, I just took off. It just fired me up to know I was about 100 percent sure I was going to Hawaii. Something came over me, and I went for it.”

To prepare for Hawaii, which is being held Oct. 18, she competed in the Lake Placid Ironman triathlon two Sundays ago, “more as an education in learning how to do these things.” Her coach, Eslick, did not want her to complete the entire event, for fear of injury. While she completed the bike and swim portions of the event, she only ran half the marathon.

“It was a tough call,” Eslick says, “but people at her level think they can’t get hurt. And that is precisely when it happens. You’re so fit you push the edge.” Sheffield herself says it took everything in the world for her to drop out of the race midway through it. At the time, she was leading some of the pros.

Eslick says his focus is on having her “peak for Hawaii,” and he describes the entire process this way: “Build, build, build, taper, destroy.” In other words, you continuously build your workouts until you get closer to the event, at which time you reduce them. Then during the event itself, you basically ruin your body.

One of the hazards of Hawaii is the wind and heat: gusts can climb to 60 miles per hour; temperatures can range from 82 to 95 degrees, and in one part of the run, a demoralizing and lonely stretch from mile 18 to mile 22, the thermometer can reach 110. Johnston says that when she competed in Hawaii, she suffered from heat exhaustion. And so she’s now going to hot yoga classes with Sheffield as preparation.

Sheffield may be relentlessly competitive when she’s in the heat of a race, but she’s approaching Hawaii with remarkable equanimity and the desire simply to compete against herself. “I can’t help but want to know what’s in me,” she says. “So my goal is to find out what’s in me. I don’t have anything to lose, but I respect that race and I respect the distance.”

She sounds even more relaxed about the world amateur championships, which take place in New Zealand at the end of this year. “I’m just going to let it all go there,” she insists. The Olympic-level distances—one-quarter of the length of an Ironman—make it possible to look at the thing as a near sprint.

She’s equally insistent about something else: “Whenever I get back from New Zealand, I’m taking a break.”

How does Sheffield have the time to train and compete and hold down her job? “I have the most wonderful husband in the world,” she says repeatedly. Bob Sheffield, who works as a policeman in the East Sector precinct, describes it as being “like a team. I try to make it extremely easy for her so that she doesn’t have to do anything but train and compete and, unfortunately, work.”

As for life in the workplace, her boss, Weaver, says associate attorneys such as Sheffield are expected to bill approximately 1,850 hours a year, which works out to about 45 to 50 hours a week. “It’s a lot. But because she’s training for the Ironman and the world championships, she and I have an understanding that she’s not going to bill 50 hours. Basically, you can imagine how hard she’s working. She’s really got two full-time jobs.”

At her law firm, Sheffield focuses on environmental law, which in odd ways parallels the work she does out of the office. “One similarity is the tremendous amount of time spent in preparation,” her husband says. Another similarity is that both require a certain anal-retentive fixation on detail. On the one hand, Sheffield knows how to piece together the fine print of environmental regulatory law. She can also tell you her heartbeat two weeks ago, or the number of carbs on your lunch plate. “I try so hard not to be anal, but I am,” she confesses.

Of considerable concern to Sheffield is the expense of her pursuit. “I’m trying to get some sponsors,” she says. Gran Fondo Bicycles in Nashville is one of two sponsors she has now. They have provided her a Tommasini bicycle. Clif Bars, which makes energy bars and those gel-liquid packets you gobble down during a race for energy, is the other sponsor.

Should Sheffield deliver on the immense promise she’s shown thus far, many are wondering in which direction she’ll head next. “I definitely think she can go pro,” says Johnston. Gennari himself is also bullish on the issue. “If Shelby wanted to turn pro, she would probably do fairly well. But I wonder if there are other things that are probably more important to her than turning pro.”

Only Sheffield—and her husband, of course—can make that decision. “I’d like to see what happens,” she says. “And I’m certainly open to the idea.” But with the brutal honesty that often accompanies marathon runs and 100-mile bike rides, she is equally blunt: “I need my day job right now.”