I believe all sports require discipline, but running especially.
Remember back when you were a new runner? Those first few (or, in my case, several) runs were not easy. My lungs burned and my legs did not want to move. And, what was that sharp, twisting pain in my side?
I wanted to be a runner, though, so I kept at it. Eventually, my legs didn't require as much warming up, and my lungs felt fine. And, best of all, those side stitches became a thing of the past.