Jen Hummel, 35, has always been a runner. After spending her high school years running cross country, she kept it up recreationally, balancing training runs and races with taking care of her two young children. “For me, it’s the best exercise and gives me time to think,” she says. But in early December, she started to experience pain and discomfort almost immediately after she’d start running. “I had pain going down my arms and in my chest. It would come up in my chin,” she explained. “I’ve always been someone who can push myself, but after I’d get up to...