I like to exercise, obviously due to the health benefits but also because of the wonderful convergence of endorphins and huge guitar riffs. Nothing makes exercise easier than loud, aggressive rock and roll...especially when you hit the wall and the only way through it is to turn the volume up on GnR's "It's So Easy", for instance.
If you are like me, and always on the lookout for good workout tunes, check out Bad Religion.
While I love them, some complain their songs all sound the same. In terms of exercise, however, it is that consistency of sound that make them perfect. You have the confidence of knowing you're going to get 30 min+ of uninterrupted guitar riffs, machine gun snare drum, and angry lyrics featuring words you need to look up in the dictionary...without any bullshit ballads to break the pace.