I started running not quite two years ago, and I'm actually kind of terrible at it: I'm not naturally athletic, I'm really slow (as in, I'd love to run a sub-30 minute 5k), and when my work or personal life gets busy, the running tends to be the first thing out the window. But I love it anyway, maybe in part because it does not come easily to me, and sometimes it's nice to work hard at something for the sake of the hard work, rather than for some extrinsic reward. (The cardiovascular health is nice, too, though.)
My big "someday soon" goal is a half marathon, not a full, but I could still identify with a lot of the points in this:
Marathon training to finish your book
Any "training techniques" you'd like to share in the comments?
See also Jessica Brockmole's terrific NaNoWriMo piece on running and writing.