Running shouldn't be your only exercise.
I'd be lying if I said I didn't know that already, but that didn't stop me from carrying on without any cross training. My legs became overworked, my back hurt all the time, and I'm sure it didn't help my recovery time after long runs.
I've recently started lifting weights, and it's already made a huge difference in both my speed and endurance. It's also important to have a stretching or yoga routine. A combination of running, strength training, and stretching is crucial for avoiding overuse injuries.