I was a competitive runner in college and it's actually the reason I won't run a marathon now. I got so burned out from that career that now I just run without a watch for fun, to de-stress. If I were to run a marathon I'd want to "do it properly" and not just finish it, and that would be way too stressful for me. Even now, when I run an annual turkey trot 5k, I can't help but race the people around me and go as fast as I can despite being terribly out of shape...