Waving the Chinese national flag above her head, face ecstatic, Yao Miao sprints through thick, cheering crowds lining the streets of Chamonix, France. It’s September 2, 2018, and the 23-year-old runner is about to win one of the world’s premier mountain ultras—the 62-mile division of the Ultra Trail du Mont Blanc (UTMB). She crosses the finish line in 11 hours, 57 minutes, and 46 seconds. The second place woman, American Katie Schide, is more than 30 minutes behind her. Despite a race that’s infamous for its high-altitude passages and grueling weather conditions, Yao Miao’s stride and posture betray no signs...