I am about two years complete for treatment of stage 3 rectal cancer at 45 y/o (47 now). Radiation/chemo first, then surgery, then more serious chemo, then another minor surgery. I worked out through all of it, other than the weeks after the surgery (which was major invasive through the abdomen). Specifics as to radiation: yes, it's a quick hit every day; at first you don't notice anything, other than maybe being tired or in some people a little nausea. As it goes on a couple of weeks, the burning sensation starts internally (wherever it is being focused). Given the location of my tumor, that made me walk around a bit slower and it became painful when I had to use the facilities. I don't know if prostate will have the same feeling, though I assume it will. In essence it is burning the tumor down from the inside. When the burning sensation hits, it sucks, but it works - - by the time of my surgery, my tumor was substantially reduced.
That said, I could still get to the gym every day and work out. I was slower on cardio because of the burning sensation internally. The burning actually lasts for 2-3 weeks after radiation, as it's still doing it's thing inside of you, and the burning part slowly works out towards the exterior where your skin becomes like a deep sunburn and in bad shape. But, through all of it you can work out AND you'll get through it. You just have to take it slow. I personally stayed focused on lifting heavy because I knew this whole process had the risk of making me lose weight, and I wanted to stay strong. In the end, I'm back to regular weight and full strength.
I wish you all the best. My wife and kids were awesome in encouraging me through it, including presenting me with maroon "BTHO Cancer" shirts at the Alabama game two years ago. The whole experience put a new perspective on things for me, but I think the personality of discipline in getting to the gym, to work, to family events , and yes to Aggie football, kept me focused to just getting it done. Sounds like the same is true for you . Your five weeks doesn't sound long, and in reality it isn't, but it may feel unending while you're in it - - trust that you'll get through it and will be stronger and better on the other end. Prayers are with you from this Ag!