“Mental tenacity — and the ability to manage and even thrive on and push through pain — is a key segregator between the mortals and immortals in running,” according to Mary Wittenberg, president and chief executive of the New York Road Runners. It’s one of the secrets of elite athletes. “But the question is, how do they do it? Can you train yourself to run, cycle, swim or do another sport at the edge of your body’s limits, or is that something that a few are born with, part of what makes them elites?”