15 million alone completed road races last year. That is an all-time high.
"There are more runners and endurance athletes out there than any other recreational sport," says Dr. Timothy Miller, a sports medicine specialist.
"And because of that, we see very specific kinds of injuries, that are, for the most part, just related to them," he continued.
Now, there's a new medical specialty for those athletes called endurance medicine.
Doctors say endurance athletes tend to have a lot of overuse injuries. But they don't want to be on the shelf for long, or they'll lose their edge.
"We not only focus on the treatment of the actual injury, we also focus on the education of the athlete and help them prevent the injury down the road.
Doctors in this area adjust treatments so athletes can heal, yet stay in shape.
For example, an underwater treadmill might let runners keep up their endurance, despite a foot injury.
Some centers also offer counseling, too, since injuries can send elite athletes into depression.