March in place, bringing your knees high and keeping your upper body posture good, with 5 to 10 pound weights on your ankles. Slowly.
Lie on the floor on your back and do straight leg raises very slowly, hold for 3 to 5 seconds, with 5 pound weights on your ankles.
Stand next to a chair or counter, hold on with one hand, and slowly move your leg to the back. Hold for 3 seconds and repeat. Have 5 pound weights on your ankles. Do the same moving your legs out to the side.
Put a firm 8 to 10" ball between your knees and squeeze your knees together. Or get one of those spring things that are made for that.
Don't neglect the muscles in your hips and trunk (back, sides, stomach). The saying is, "Stability before mobility." You need a stable foundation before you can use your legs and arms well.
And, finally these suggestions are assuming you are in good enough shape to exercise. The other saying is, "Always check with your doctor before starting any excercise program."