In 2007, Anne Mahlum was living in Philadelphia. She had a habit of waking up early to exercise; each morning, her route took her by a homeless shelter. Even at dawn, there were people out in front. One morning, they waved. As Mahlum built up a rapport with her early-morning neighbors, she started thinking: Why am I running past them when I could run with them? Mahlum had started running as a teenager to cope with troubles at home. Maybe, she thought, running could help other people the same...