Say hello to your Tibialis anterior V.

It's the muscle that controls the speed at which your forefoot drops after your heel has contacted the ground. And yup, it hurts like an SOB if you've been away from running for awhile. It also hurts more if you're landing with your foot too far out in front of your body, or hitting too heavily at the heel. So you might want to think about those things the next time you go out for a run.