As a woman who gave birth to a big baby boy without the dulling effects of an epidural or any other drug, I can tell you that it is a very painful process, but there's a certain amount of tolerance that one can develop for the pain when one focuses on the outcome rather than suffering. I would imagine that great runners, male or female, have an ability to withstand pain that exceeds the great majority of their competitors. This is certainly coupled with talent and an intense drive to succeed, but I'd venture to guess that the greatest runners are able to tolerate the pain of intense workouts because they focus on the end result rather than whining about how much the immediate physical output is hurting them...
But just so you know, women ARE way tougher than men. My husband had to take a nap while I was in labor because the stress of it was too much for him to handle! Imagine what would have happened to him if he had to physically endure an actual contraction! (-;