If you, like me, spend a lot of time running (on the treadmill or outside) then you're definitely giving your hamstrings a nice workout, which is awesome. Keep in mind, though, that it is important to work opposing muscles in order to keep your body alignment and balance in check. This does not mean you need to start running backwards (unless you want to, of course). Instead, just be sure to work your quads (the hamstrings opposing muscle group) a few times a week and doing so should help balance out all the great running you're doing.
And in case you're wondering . . . a good, no fuss exercise that is a surefire way to work your quads is squats.