Moon Fahel from San Antonio, Texas, had just gotten home from a martial arts class that he’s been taking for the last 15 years when he answered the phone call from Runner’s World. It wasn’t an anomaly—he stays busy with other classes during rest of the week, too: kickboxing, Zumba, athletic dancing, and BODYPUMP, among others. Plus, he just ran his fifth marathon this past Sunday at the San Antonio Rock ’n’ Roll Marathon. Oh, and he’s 85 years of age. (The word “old” doesn’t have a place in his vocabulary.) “I don’t know what old is,” Fahel said. “If...