This may well be some simple patellofemoral joint pain. If it was your first ride for a while then this is not uncommon. Also if the ride was a bit tougher than usual this may also be a reaction to normal increase in loading in a 56 year old. Nothing to worry about but often these things can be cummulative and that overtime they become painful. If you think that may be closer to the reality then having someone chck you out and chck your bike setup would be useful for you in the longterm. Happy MTB'ing