iTunes has some really nice - and FREE - podcasts of music specifically mixed for working out, in all kinds of genres - and it's not just playlists, it's mixed so that there's no breaks between songs, and some of them are really carefully designed to speed up and slow down for running programs, or to provide warm-up, workout, and cool-down music for you. New ones come out every week, so there's always something new so I can avoid treadmill-music-burnout.