It is normal to develop some soreness anytime you use a muscle group vigorously or for a prolonged period. Soreness in your legs that develops immediately after running usually fades over the course of several hours. Muscle soreness that develops more than 24 hours after running is due to tiny tears in your muscle fibers. This condition is called delayed onset muscle soreness, or DOMS. DOMS is normal and may take several days to improve with home treatment. Report any sudden or severe pain in your legs after running to your doctor.
Stop running or engaging in any other activity that works your leg muscles for at least one week. This gives your muscles time to heal. You can continue working other muscle groups, such as your back, shoulders and arms.