Last summer, Kaitlin Gregg Goodman felt poised for another breakthrough. She’d just signed a new contract with the Boston Athletic Association High Performance Team and Adidas. Her training for the New York City Marathon was clicking—enough, she hoped, to improve on the 2:32:08 personal-best she’d set at the 2017 California International Marathon the previous December. All that changed in an instant. At the end of an easy second run one early August evening, about a quarter-mile from her Providence, Rhode Island, home, a distracted driver nearly struck her. She leapt to safety, but the fall partially tore the tendon attaching...