Daniel Lieberman and Dennis Bramble in 2004 found that the human body is designed to optimize long-distance running. Homo sapiens are not meant to outrun the deers and mammoths- they are meant to outlast them. Humans can cool down by sweating. The Achilles heel acts as a spring. This made the cover of Nature.
So running a marathon (or the desire to do so) is biologically natural.
I think I need to work on the psychological part of running. Here's my thought process when I am running: This is hard. I really want to stop. I could stop for twenty seconds. Okay, whew, that feels better. Why am I even doing this? I could walk a little bit more. Ooh, that guy just passed me. What? I'm gonna outrun him then walk a bit more. Yes!!! Oh, I'm tired...
When I actually stop whining and examine myself, I'm not even that tired physically. So the trick is to keep going (in my mind). I imagine this is harder when you are doing this for four or five hours. I did cut down on my time last Saturday although I stopped and walked four stretches. It was awfully hot in Houston and I'd underestimated just how humid it would be at 8AM.
But this kind of self-discipline must be useful in other aspects of life. For athletes, their greatest achievement isn't just what they do physically, but that they can push themselves in training and in competition to give their 110%.