My friend, who is a terrible athlete, recently decided to run a half marathon. When I say she is a bad athlete I don't mean she doesn't play sports. She plays tennis, maybe a couple times a month. However, she is not particularly coordinated and I didn't think she would have anywhere near the needed endurance to complete such a long run.
To my surprise, she trained and eventually completed her first half marathon. There were other runners there who were better athletes and in better condition than my friend who were unable to finish the run.
What made the difference? My friend is a person who never gives up. She is driven and successful in most areas of her life. I think it was her mental fortitude that allowed her to do what she physically should not have been able to do.