What causes severe pain in the calf and thigh muscles of both legs?

Lots of things: Severe calf pain can be due to intermittant claudication which you must address due to poor circulation. Upper thigh pain can also be due to poor circulation especially if you are a past smoker. Overexercise which causes cramping, and lack of stretching may also be a cause. Arthridities and neuromuscular disorders should also be addressed. I would see your physician for a proper diagnosis.