When Suzanne Stone completed her first half-marathon in February, the achievement was extra sweet: she’d managed to finish the long-distance race two years after being diagnosed with a brain tumor.

Suzanne had been on her usual morning run the week before Thanksgiving 2014 when she started to notice the symptoms: “I felt really, really funny and my words were all garbled,” Suzanne says. A colleague drove her to the ER, where an...