The town of Reston will be the site for one of the world's fastest- growing participatory sports -- the triathlon -- on Sept. 9.

The triathlon has spawned a whole new class of athletes, who push themselves to the limit in three endurance events back-to-back, usually swimming, bicycling and running, in search of that feeling of accomplishment, a "high" to top all "runner's highs."

The Reston Triathlon begins at 8 a.m. with a one-mile swim, out and back, in Lake Audubon. After completing the swim, the athletes will change into bicycling clothes and pedal a hilly, two-loop, 23-mile bicycle segment starting at the Lake Audubon Dam and finishing at South Lakes High School. The final event, a 6.2-mile run, will follow an out-and-back course along Reston's bike trails starting and finishing at the high school.

The event was organized by the Reston Masters Swim Team, the Reston Bicycle Club and the Reston Runners in response to the growing demand for triathlons in the Washington area. As it stands now, athletes must travel hundreds of miles to compete regularly in the three-event races.

"I think this triathlon will be the first of many in Reston," said Bobbi Jennings, 41, a part-time nurse and mother of four. "Reston has had to turn people down by the droves."

Only three weeks after the Reston Triathlon Association announced the race last November, 350 athletes requested registration forms. At first, the association decided to limit the race to 125 people because they "didn't want to overcrowd the lake swim," said John Davison, director of registration.

But after conferring with the director of the swimming event, the association agreed to allow 200 athletes to compete. There still remains a waiting list of nearly 100.

A large number of the athletes, more than 65 percent, live in either Reston or Herndon. "In Reston, we have a running club, a biking club, and a masters swim team. It is a very athletic-oriented area," commented Davison on the large local response.

The fact that such a large percentage of the participants hail from the Reston-Herndon area makes the Reston event unusual among triathlons across the country, which usually draw a large percentage of athletes from outside the area of the event. But the local race has given at least half the entrants a chance to compete in their first triathlon in their own back yard.

"The athletes here are excited about the race," said David Heymsfeld of Reston, a 46-year-old lawyerfor the House aviation subcommittee, attempting his first triathlon. "I see a tremendous number of people trying new things. People are biking into the city to work to train for this race. (It's 40 miles round trip.) The swimmers have been running on the roads. It has brought the three clubs together. I've met people who swim whom I've never known before."

Triathloning has blossomed at a time when many Americans are becoming increasingly health- and fitness-conscious. After the running craze took root in America in the early 1970s, the 26-mile, 385-yard marathon became a major athletic buzzword.

Swimming the English Channel and bicycling across the country was satisfying enough until athletes realized that these feats could be accomplished. Then they needed something more challenging.

The search for greater conquests prompted Navy Cmdr. John Collins in 1978 to bet a handful of his beer-drinking friends that they couldn't swim 2.4 miles in the ocean off the Hawaii coast, then bicycle 112 miles through the lava fields of the island, and finally run a marathon. This most famous of triathlons, the Ironman Triathlon, has taken place in Hawaii every year since then, representing the ultimate endurance test that only the most physically fit competitors finish.

But athletes intrigued with the three-event idea who could not withstand the distances the Ironman requires began organizing scaled-down Ironman races called Tinman triathlons. In the past six years, thousands of athletes have competed in more than 1,000 triathlons run every year in this country.

As with preparing for a marathon or century (100 miles) bicycle ride, Reston participants say finding the time to train for not just one but all three events can be difficult and demanding.

"My husband will really be glad when it's over," explained Jennings, who has been training for her second triathlon during the day while working the midnight shift at Fairfax Hospital. "It's really been hard, and the family has had to adjust. I'm fortunate that I don't have a full-time job."

Heymsfeld has run eight marathons. "With all the years I've been running 12 , my family is used to me exercising. I try to run at times when it doesn't interfere with the family his wife and two sons , like at lunch."

And why do these athletes participate in such an event? "It's a new challenge," said Heymsfeld. "The marathon gets to be old hat, the same training program. It's an incentive to improve my swimming."

Jennings said she does not see herself as an athlete, even though she has run two marathons and one triathlon. "I consider myself to have no athletic ability. I'm not coordinated and I'm not skilled. I'm not an athlete. But . . . I believe that anyone can do them if they are determined."

The Reston Triathlon joins similar triathlons in Arlington, Richmond, and Oxford, Md., and has been designated as a qualifying event for the 1984 Invitational Triathlon Federation short course national championship Sept. 22 at Bass Lake, Calif. The top 10 male and female finishers at the Reston race will make the finals.

The association consists of a group of triathletes who have traveled from as far as Massachusetts to participate. "The core of the group are triathletes, using their expertise in planning the event," said Davison, a 37-year-old planning analyst at Reston Land Corp. and three-time triathlete. "We have a prime place to have a triathlon in the Reston area, with the rolling hills, shaded pathways, and beautiful lakes."

The triathlon, planned to coincide with Reston's 20th anniversary, is the first of its kind in Reston, and, according to Davison, it is the only triathlon in the Washington area with an open-water swim, as opposed to swimming the first segment of the race in a pool.

Athletes will be competing for awards in eight age groups for males and females as well as the first male and female finishers.