As an endurance athlete pain is par for the course. Not the pain of injury, but the sweet pain of progress.
I stumbled on an interesting article from the New York Times, that discusses pain and discomfort during exercise, and what makes the athlete stop.
Also, it brings up the euphoria that most of us have felt at one time or another. The feeling you have after a hard interval session, or a long run. You lay on the couch completely spent, and elated at the same time.