I tore my meniscus in January and just went back to riding last week. I've been in physical therapy for over six months (and still am). I've finally overcome my knee injury, so my physical therapist is now aiming to strengthen the muscles in my legs. One of the exercises I perform at physical therapy requires a pillow and a wall.
Take the pillow and fold it in half. Place it between your thighs. Then, with your back against the wall, slowly slide down until you are in a sitting position, squeezing the pillow tightly. Hold this position for 15 seconds and then come back up again. Trust me, it burns, but it really helps.