Morning Carl.
I can only give you my opinion, but totally understand your dilemma of having your knee surgery during this prostrate treatment period.
My treatment of surgery happened about 6 weeks post m d t meeting, I had my surgery in Wakefield west York’s but as we know different postcodes have different timescales. Perhaps the best thing would be to speak to either the surgeon doing your knee surgery or the surgeon/oncologist in charge of your prostrate treatment. You of course have to decide on the treatment you wish to have i e surgery or other. Although prostrate cancer is slow spreading my option would be to sort the prostrate issues out before the knee if that was my only choice, to give you a guide on my recovery I was fully continent after about a week, walking around the village 2 days after leaving hospital then steadily increasing my activity over the next few weeks & back to my usual running of 3 miles a day at the 6 week period. I think radio therapy is over a longer period of about a month of treatments. On your back ache problems at our ages of 68/69 backache can be just a wear & tear issue & totally nothing to do with the pc. Until your meeting with the specialist, the treatment they offer & of course what treatment you decide on, things cannot be decided.
Best of luck with whatever you decide.
Jeff.