Hi Chris,
Good that RT is starting soon. How many sessions? Probably 15 or 20 or 37 seem to be common numbers. Travelling to hospital every day can be inconvenient. I only had 15 and I was able to go by motorbike without problem. I think if you have a greater number of sessions, you can start getting caught short with needing the toilet. Some people report fatigue.
With the cancer having spread I think your doctor thinks he will never be able to eliminate it completely, so it sounds like he thinks this treatment will hold it at bay for 3 to 6 years. Then it will be a case of trying another treatment, could be more RT or hormone therapy or chemo or a host of other treatments. It is pointless trying to guess what the next treatment will be it depends on if and where you get a recurrence of this cancer.
There are people on here who have lived a long time with advanced prostate cancer by moving from one treatment to another and new treatments are being trialled all the time.
There are even people on here who have refused treatment and carried on living for many years. Personally I would just follow the advice of the medics, but I have great respect for someone who is prepared to go down their own path.
So in short the medic is not saying you only have three to six years to live. He is saying you will need further treatment in three to six years and that is too far in the future to be able to say what it will be, and then how long after that you will need even further treatment.