I agree with Velo. In my riding, I don't count cadence, I just do what my body and after 40 years in the saddle, my instincts are just fine. The key is don't push it. No pain no gain leads to big pain and wrecked bodies. I have had the retraining and physical therapy to prove it.

Another thing, my knees had deterioratied so much by the time I was thirty five it showed on x ray. Fortunately, thanks to a great riding partner (who is also my husband)I was able to learn to warm up gradually and reduce the stress. Now I can truly listen to my body in shifting.