As I've been riding more and longer in the past few months, I've noticed that after I get home after a ride my knees feel pretty sore. I only ride walk/trot for about 45 minutes maybe three days a week, but I do plenty of posting and practice two-point. I have a family history of bad knees (and sometimes my knees will bother me for no reason) and I don't want that to interfere with my riding future (I'm only 16). I don't feel like my knees are getting stronger as I ride, which I had hoped. Instead I feel like they're being worn down.
Is there any low-impact exercise that I can do to strengthen them? I like to swim but as winter approaches, I won't have that much longer.