FIVE AND A HALF years ago, Chris Harris, a 33-year-old private-equity director in San Francisco, could barely swim a single lap. Last summer, he completed his second Ironman triathlon—a 2.4-mile swim followed by a 112-mile bike ride rounded out with a 26.2-mile run—and is contemplating a third. "I've loved sports my entire life, but I've never been particularly gifted," said Mr. Harris. But with the help of a heart-rate monitoring watch, cutting-edge wetsuit, carbon-fiber bike and grueling training regimen, he managed to finish his last event in an impressive 10 hours and 27 minutes, ranking 217 out of 1,929...