What makes leg muscles sore and weak?

Deconditioning. Deconditioning, also called disuse atrophy (due to sedentary lifestyle and physical inactivity) is the most common cause in north america. Other causes include vascular (blockages of arteries down the leg) or spinal stenosis (tightening around spinal nerves). If not better with daily running - walking - stationary bike - rowing machine use for 6-8 weeks, have your family doc start the workup.
Fibromyalgia. Although this is a "catch all" diagnosis in a lot of cases, one of the common reason for diffuse aching and soreness in muscles can be lead to a condition called fibromyalgia. Consulting a reumatologist would help.