I'll add this comment about overdoing it, I recently did a reasonably long trip, on a relatively heavy bike in fairly hilly terrain and by being careful with shifting and being reasonable with daily distances, my knees were generally ok.
Back home I've been mostly commuting again, and I find it almost funny that due to the fact that I often only ride for 30 mins at a time, but hammer it at times, my one knee is complaining a bit now.

We have to listen to our bodies, pure and simple.