By the time she was in eighth grade, Mary Cain could run faster than almost any girl in the United States. A tangle of endless limbs and curly hair, she would blaze around the track as if on wheels—as if leaving cartoon trails of smoke and fire behind her—then collapse into a terrific, goofy smile. At 17, while still a student at Bronxville High School outside New York City, she represented the U.S. at the 2013 World Championships, becoming the youngest American ever to do that. The next year she was the World Junior Champion in the 3000 meters. In 2013, as a high school senior, she turned pro, signing with the Nike Oregon Project , the elite professional track and field program run by Alberto Salazar . Just three years later, in 2016, she returned home to New York and announced she was no longer coached by Salazar. Injury cut her 2017 season short. But in 2018, she told Runner’s World , “Never lose sight of who you are...…