Kristina Baum, 37, had an inauspicious start to distance running: She signed up for her first race, the Army Ten Miler, back in 2006 with zero training. “I thought I might as well just show up and try it, and see as far I can get,” she tells Runner’s World. She ran eight miles without stopping and walked the last two. The physical after-effects were rough—she threw up twice, and was in pain for days—but finishing the race completely hooked her on running. So she started training, and ran the Houston Half Marathon in 2007 and 2009. But at the...