I also am a Spinning instructor. I think three classes for most people is maximum. A great adjunct to cycling is running and/or fast walking. Spinning uses your quads heavily. Running/walking will use your hamstrings.

My concern for you is that by the time you acknowledge a problem arising, e.g., knee or hip issues, it may already be a chronic imbalance. Notice I said "acknowledge" because often we think these little aches will go away with time. Sometimes they do but sometimes they become bigger issues that plague us.