Kristen Dollard, a yoga instructor in New York City, didn't consider herself an athlete in the traditional sense. But that was before the 29-year-old spent the past year in intense training to compete in her first triathlon (search) last April.

The hard work paid off. Dollard, who had selected for her maiden run an Olympic-length race sponsored by the Leukemia Society in St. Petersburg, Fla., completed all three legs of the running, biking and swimming contest. She said crossing the finish line made it the best day of her life.

"It wasn't just a rush, but a real sense of pushing yourself — the journey and the training were just as enjoyable as the race," she said.

With summer in full swing, Dollard has plenty of company among women who have literally hit the ground running with a new approach to fitness. Endurance sports like long-distance running and triathlons are seeing their biggest increase in female participation than ever before, as women are trading in their aerobics classes for running shoes and mountain bikes.

According to the Road Runners Club of America (search), though overall membership has stayed steady since running peaked in the 1980s, female membership has been on the rise. The New York chapter of the club has seen female membership jump from 31 percent in the 1980s to 45 percent today.

"We're seeing a small increase in pockets of membership, particularly a big increase in the number of women running marathons and longer-distance races," said Becky Lambros, the executive director of the Road Runners.

For example, a majority — 52 percent — of the runners in the Cherry Blossom 10-miler in Washington, D.C., this year were women.

Denise Tarrant is one of the new converts. A former aerobics instructor, Tarrant, 46, of Evansville, Ind., completed her seventh 13 miles half-marathon in May. A busy working mother, Tarrant said she switched to running because it was a better fit with her lifestyle.

"It's easier to run than to try to follow an aerobics class schedule," Tarrant, who squeezes her workouts into morning runs, said.

The running world has taken notice, organizing more women-only events like the More marathon for women over 40 in New York City and triathlons like the Danskin series that host all-women swim-bike-run events across the country.

The Danskin Women's Triathlon Series, the longest-running series of its kind, began in 1990 with three races and 150 participants. In 2005, Danskin will host eight races and expects 5,100 participants.

Triathlons are not just exploding among women. According to USA Triathlon (search ), membership has grown from nearly 16,000 in 1993 to over 53,000 in 2004. The number of U.S. triathlon events has nearly doubled in the last six years to well over 1,000 events annually.

What's more, USA triathlon estimates that 150,000 to 200,000 people in the United States try a multi-sport event each year, whether it's a triathlon, duathlon (run, bike and run) or aquathons (swim and run).

But it is female membership that has increased the most, from 11 percent in the early 1990s to around 29 percent today.

Dollard said she was attracted to triathlons by the fact that competing was going to require her to teach her body brand new skills.

"I don't consider myself athletic at all," she said. "I really wanted to learn to run. I did swim, and I'm a yoga instructor, but the triathlon requires endurance."

Dollard said she was also tired of rooting from the sidelines.

"My brother races pro and I was so inspired after watching people of all shapes and sizes competing. It was such a rush from the sidelines," she said.

Watching others compete, she said, made her realize that "these are my prime years and I'm out to do as much as I can before I can't do it anymore."

Surprisingly, both Tarrant and Dollard said they find much more camaraderie and social support in marathon and triathlon training than they did in classes jammed with students.

For her event, Dollard signed up for the Leukemia Society's Team in Training to prepare for the race with a group. Tarrant runs with a group of women each morning.

"It's a freeing opportunity to talk and lift each other up with encouragement which wouldn't happen in an aerobics class," Tarrant said.

Gyms and health clubs have felt the shift and are adapting their programs to meet and encourage their members' new interests. Kathie Davis, the CEO and executive director of IDEA Health and Fitness Association, a membership organization for aerobics instructors, personal trainers and other health and fitness professionals, said fitness directors now advertise and arrange training sessions for local running events. People who go to the gym, she said, still turn to indoor treadmills and bicycles to prepare for races and will find plenty of support at their health club.

"A lot of women might have never thought of doing a marathon, but through their affiliation with a personal trainer or fitness center, they're now in shape enough do one," she explains.

But what is it about triathlons that are making them so popular?

B.J. Hoeptner Evans, manager of media relations for USA Triathlon, believes the increased participation has been helped by the television exposure the event gets.

"We're definitely helped by the Olympics and through the Ironman on NBC, the way they package it to focus on individual stories," Evans said. "There are more opportunities to participate in events."

Fitness experts say there is also a spiritual and mental benefit to these events. In addition to testing one's physical endurance, competing in and completing a triathlon can provide an incredible sense of accomplishment.

"Part of the increase in these competitive events is that it's an aspiration type of exercise that gives a sense of accomplishment," said Dr. Cedric Bryant, chief exercise physiologist for the American Council on Exercise.

There's no question there. The Ironman, the king of all triathlons, requires participants to swim 2.4 miles, bike 112 miles and run a full marathon, 26.2 miles. The Olympic or international distance — the length of Dollard's event — includes a 1500-meter swim, a 40-kilometer bike ride and a 10-kilometer run, and is the most popular.

But there are also many smaller events, sprint-distance races consisting of anywhere from a quarter-mile swim to an eight- to 15-mile bike ride and three-mile run. The number of these events offered throughout the year has also increased, offering something for everybody.

"Some people look at the triathlon as just something to finish in a supportive environment with less focus on winning," Evans said.

Dr. Bryant said that there are also many practical and functional reasons that people are turning to triathlon training. One can pretty much run or bike anywhere, at any time of day, eliminating the time limits and inconvenience that many people offer as their excuse for not exercising. And training for three different activities can eliminate boredom and monotony, work a broader range of muscles, and prevent over-training injuries that result from spending too much time doing just one thing.

And for many women, this means they're soaring to new heights in fitness.

"We can say without a doubt that more women are becoming active and choosing things that are appealing to them," said Davis.