I had double knee replacement surgery about ten years ago and have found that the stationary bike is best for me. Could be that walking prevents dementia and biking does not, but I don’t want to wear out my knees again. Replacement knees are a miracle, but they are no match for OEM.
Did you used to do a lot of walking before your knee replacements? It is something that I’m sure I need to watch out for as we get older.