This metaphor doesn't come from me, but rather from Elizabeth Haugen, the pastor of Washington Plaza Baptist Church in Reston. She compare writing to running a marathon.
I was a runner, once, in my twenties and thirties, although walking for exercise is now more my style. I actually ran in a race, a 10K in which I came in dead last. I was never fast, but I was tenacious.
Enjoy Elizabeth's post, "Like Running a Marathon...Writing a Book," at http://preacherontheplaza.wordpress.com/2012/07/09/like-running-a-marathon-writing-a-book/