IT'S not enough just to be a good swimmer or a competitive bike racer or a long-distance runner. To be a triathlete, you must be all three. And two of every 10 such all-around athletes today are women.Of the 490 triathletes who participated in the recent 10th anniversary Bay State Triathlon in Medford, Mass., a long-distance triathlon, only 44 were women. But in one of the newest and toughest tests of athletic ability, the number of female competitors is steadily getting larger, particularly in shorter races, says Lew Kidder, co-publisher of Triathlon Today magazine. According to Mr. Kidder, women are looking for more than "a macho event." Most women who enter triathlons are not as interested in high-intensity competition as they are in the opportunity to meet and train with other sports-minded people, compete on their own level, and, as one devoted triathlete says, "find out how much they can do." Karen Smyers, winner of last month's Bay State triathlon and the Danskin Women's Triathlon in June, is at 29 years old a National Champion and International Triathlon Union World Champion. Shorter races are not as rewarding financially as some larger events: A national series race winner might receive between $1,000 and $10,000; the male and female winners of the Hawaiian Ironman each receive $20,000. The top five finishers in Medford shared about $2,000 among them. But Ms. Smyers tries to race the short er ones as often as possible to show support for triathlons and the women who are trying them for the first time. "I think it's important that the elite [professional] women take part ... to be role models and motivate the girls to stick with it and give them something to shoot for," she says in an interview. The triathlon - a swim, bike, and foot race of varying distances - has come a long way since the first Hawaiian Ironman Triathlon in 1978. That grueling race, consisting of a 2.5-mile swim, 112-mile bike ride, and 26.2-mile run (marathon distance) inspired a number of athletes to take up the sport, says Smyers. Still, "unless you're a nut case," watching an Ironman race does not inspire the average athlete to compete in one, says Smyers. "For every one Ironman, there are 50 shorter races that are so much more suitable for most people," she says. Many of the shorter races are Sprint distance (less than 1/2-mile swim, 15-mile bike ride, 3 1/2-mile run), Olympic distance (1.5K swim, 40K bike ride, 10K run), or international-distance races (anything longer than a Sprint). The number of women competing in shorter triathlons is growing, says Tim Yount, spokesman for the Triathlon Federation/USA, the national governing body for triathlons and biathlons (two-event races). Mr. Yount estimates that five or six years ago only 15 percent of triathletes were women; that figure is now about 20 percent. "Women are becoming more involved and more vocal in the sport," he says. "They're saying, 'Let's try to do these things so we can benefit women athletes all over the country. Smyers, a Princeton University graduate and former computer consultant, began competing professionally in 1984. This October she will defend her World Championship title in Australia. She says American women already have an advantage over many of the European women racers. "In those countries, only 5 percent, maybe less, of the field will be women," says Smyers. "In some cultures, women aren't supposed to sweat and be fit and spend time on such things. Here, people will see a woman out running and think she looks beautiful." Still, it's important to get girls involved, says Smyers. In the Danskin race, 12-year-old Katy Radkewich of Hudson, Ohio, held her own against some of the top racers and finished first in her age group, 40th overall out of 750 racers. "We've got to have more meets out there for girls to compete in - to get them used to seeing themselves as both athletic and competitive," Smyers says. But triathlons are gaining ground not only among younger women. Margaret Regina, an amateur triathlete from Reston, Va., raced her first full-distance triathlon two years ago at age 50 a time when most people start to slow down," she says. Ms. Regina started doing triathlons at the urging of some friends, and stuck with it because she found a built-in support system among her racing peers. Smyers says that most of the women involved in triathlons are not laid-back, nor do they race simply because they love being outdoors. "Most of the women are interested in how well they're doing against their competitors and in improving their times," she says. "In a road race you might find some people out simply for a jog or just to take part in something. I wish more triathletes would do it just to participate." Beth O'Connor, a 30-year-old teammate of Regina's on a Masters swim team in Reston and a former varsity swimmer at the University of Virginia, says that women who participate in triathlons tend to be "driven, successful, and competitive." Ms. O'Connor says many women like herself started doing triathlons out of the feeling that they needed to do something for themselves and even to prove something to themselves. "I try to keep it [triathlon training and competition] in perspective," she says. "I know peo ple who have gotten divorced over triathlons because they devote so much time to it. I like to think I have other things in my life." O'Connor says she prefers the shorter races because they are easier to train for and she is less likely to lose interest. She usually trains six days a week for several hours a day, but says she's gotten to the point where she isn't concerned about missing a day here and there. Mr. Kidder of Triathlon Today magazine says he believes that a majority of women prefer shorter races because the training fits in better with a multifaceted lifestyle. Smyers, who after her win in Medford planed to compete in seven races over seven weekends leading up to the World Championships, says that if a woman is to be successful as a triathlete and continue to enjoy the sport, she must learn to relax and not take the pressure too seriously. Smyers had to find this out for herself after not doing as well as she would have liked in her first few years as a professional triathlete. "I had to stop spinning my wheels and trying so hard that I wasn't going anywhere," she says. Judging from the smile on her face as she led the pack out of the water at the Bay State races, she's learned to do just that.