Typically when I run, I've been running to hard music like Metallica as well as groups like The Killers and Flogging Molly. But I've found that listening to Johnny Cash is very relaxing, particularly on long runs.
I'm not sure how Johnny Cash would work on shorter runs, when you have to exert yourself more and the more upbeat or harder songs are better suited for such a workout.
But when you are in for the long haul, for 90 minutes or more of running, you need something that will take your mind off of things, to help you relax, some sort of soothing music without it putting you to sleep. Johnny Cash fits the bill for me.
I particularly love this song, have for a while but now it's got some new-found life to it.