Jason Houston, a 32-year-old performance coach who runs in New York City, did his first triathlon on a dare in 2009. "I figured it couldn't be that hard, so I didn't train much, wore baggy shorts for the swim and tennis shoes for the run," he says. "I came in last among my friends." Houston now regularly places near the top of his age group and competes in sprint and Olympic distance races. He intends to do an Ironman triathlon: That's a 2.4-mile swim, a 112-mile bike ride and a 26.2-mile run. About two million people competed in triathlons in the U.S. in 2011, according to the Sports & Fitness Industry Association. USA Triathlon, which oversees races, recommends first timers start with the "Sprint" which includes a half-mile swim, a 13-mile bike ride and a 3-mile run.