No harder question than, "Why do you run?" Scott Jurek's book Eat And Run nails it for me on page 6:
"As a child I ran in the woods and around my house for fun. As a teen, I ran to get my body in better shape. Later, I ran to find peace. I ran, and kept running, because I had learned that once you started something you didn't quit, because in life, much like in an ultramarathon, you have to keep pressing forward. Eventually I ran because I turned into a runner, and my sport brought me physical pleasure and spirited me away from debt and disease, from the niggling worries of everyday existence. I ran because I grew to love other runners. I ran because I loved challenges and because there is no better feeling than arriving at the finish line or completing a difficult training run. And because, as an accomplished runner, I could tell others how rewarding it was to live healthily, to move my body every day, to get through difficulties, to eat with consciousness, that what mattered wasn't how much money you made or where you lived, it was how you lived. I ran because overcoming the difficulties of an ultramarathon reminded me that I cold overcome the difficulties of life, that overcoming difficulties was life."