Seven years had passed since Mena Hawkins had attempted the Las Vegas Great Santa Run. Her first attempt at the one-mile race ended after the streets had already been reopened, leaving the then 10-year-old to finish the race on the sidewalk. Her final time that year was 1 hour and 20 minutes. Despite finishing well behind the rest of the field, Hawkins wasn’t defeated by her effort. The Henderson, Nevada, native—who is now 17—has cerebral palsy, a congenital disorder that affects movement and muscle tone or posture, which has always made walking a serious challenge. In school and most places,...