Arriving, you see them, the Iron People, cycling on Kona’s Queen K Highway in one-piece triathlon suits and aero helmets, these pilgrims’ ceremonial clothes. The Ironman World Championship is the hardest major race in the world: 2.4-mile swim, 112-mile bicycle ride, 26.2-mile run, all in the shadeless tropical heat. Yet the event is filled with unlikely apostles: mothers of young children, three-limbed amputees, octogenarians, all ticking Kona off their otherwise divergent bucket lists because of a fascination for what’s difficult. Because marathons have been ruined by people who think it’s fine to walk. Because life is too easy and Everest is too far away.

The Ironman was not supposed to be for everybody. It was supposed to be for nut jobs. Twelve people finished the first, in 1978. Fifteen people competed in the second. Then, in 1982, Wide World of Sports aired pretty, limby, 23-year-old Julie Moss stumbling like a newborn giraffe across the finish line—muscles spent, personifying the ­limits of what’s possible—and the race jumped. These days, each of the 27 worldwide Ironman events sells out in minutes. Ironman Asia-Pacific sold out in five. The inaugural Ironman U.S. Championship (in New York City and New Jersey) sold out in just over 11. The 60,000-plus race slots available in 2012 won’t begin to slake the demand.

The 1,855 berths at the Ironman World Championship are the most coveted. The event takes place in early October on the kona—translation: leeward—or dry side of Hawaii’s Big Island, in the mellow tourist town of Kailua-Kona. The swim is out and back from Dig Me Beach, a horseshoe of sand just off Alii Drive, Kailua-Kona’s main street and the course’s finish line. Both the bike and the run take turns through town, then head north on Queen K Highway, just inland from the island’s west coast, where the monot­ony of the black lava fields is broken only by racers’ names spelled out in small pieces of white coral. At...