I was just talking to one of my running friends about this topic when it came up on a blogpost I just read - running helps our mental state!
If you are a runner or any sort of fitness enthusiast . . . it really is true . . . your fitness activity helps your mental fitness as well. I don't know that I would know what to do if I couldn't identify some fitness actitivy to help keep me fit and sane.
The irunfar.com blog recently featured some reflections on this issue. See if they match up with your perception of running and fitness.