My thighs are easily fatigued during exercise?

This is actually and on-going condition I've had pretty much since I can remember, but it's horrible right now because I haven't exercised properly in 2 years. Whenever I run on a treadmill or elliptical, or do the stair-stepper, my thighs feel like cement. My calves end up taking the brunt of the work. I'm not sure if this is something serious that I should have checked out. I don't have time to go to the doctor for something like this. It just makes exercising or running very difficult and short-lived because I can only do the stair-stepper/bicycle/elliptical for 10-15 minutes.