[sidebar] One of the best ways to stay fit and injury-free in your 30s, 40s, and beyond is one many women have already discovered: triathlons—a race where you swim, bike, and run, one sport immediately following the other. Nearly 40% of triathletes are now women, and female participation has jumped 10% over the last decade, according to USA Triathlon. What’s more, the largest membership increase is within the 30 to 49 age groups, and the average triathlon participant is 38.

 Why the buzz? “It’s essentially forced cross-training,” Holland explains. Because triathletes need to spend time in the pool and on the bike, their joints and muscles get a break from the constant pounding and stress of running, which makes you less likely to experience overuse injuries like shin splints. Regularly varying your training also helps keeps your metabolism high: “You’re constantly forcing muscles to adapt to new movements, which works them harder than if you were doing the same activity day in and day out, so you can burn more calories,” Holland says. (If it’s been a long time since you had any fun while exercising, check out these 5 Fun Fat-Blasting Workouts.) 
Nicole Donovan, 45, of Connecticut, has personally seen tri training change her life. The former high school gymnast and mother of six first started working out a few days a week at 43 to get fitter and lose weight. To stay on track, she decided to sign up for her first triathlon. “There were some tidal issues with the swim, and over 1,000 participants,” she says. “But it didn’t affect me. I was so happy just to be there. My goal was to finish. And I did. I felt fantastic.”
 
Now she’s a triathlon regular. “My body has changed back to the one I had when I was a gymnast,” she says. “I constantly get compliments for my shoulders and arms thanks to all the swimming.” (You can get sleek, sexy arms and shoulders too, with these 10-Minute Arm-Toning Exercises.) Ready to follow Donovan’s lead? Here are a few things you need to know about triathlons before you get started:

Make your first race a sprint triathlon. This is the shortest distance—usually a half-mile swim, 12-mile bike, and 3-mile run. Most beginners will finish the race in about 2 hours.The average training plan for a sprint triathlon lasts about 6 to 8 weeks.You don’t have to invest in a ton of fancy equipment. You can ride any working bike—yes, even a mountain bike or beach cruiser. Many local bike shops will rent you a bike for a day, andWomen’s races tend to be especially beginner-friendly. Check out the Athleta Iron Girl series or search trifind.com. And invite your best gal pals to join you!If you haven’t swam the freestyle stroke since high school (or ever), look for a group adult swim class for triathletes. You can usually find one through a local triathlon club or a local pool. Many triathlon coaches also offer one-on-one swim coaching.A triathlon is a logistical marvel. Make sure to read the race website and athlete packet thoroughly so you know exactly where you need to be and when, and so you’re familiar with triathlon rules.

More from Prevention: Sign up for our free Feel-Your-Best-Fitness newsletter