Im a Radiation therapist, the fatigue people speak of is pretty common with radiation. Its been a minute since I looked at the literature but generally our recommendations are do what you can but listen to your body. Ive had patients that have spent an hour in the pool every day while on treatment, it can be quite varied depending on how much tissue is being treated and how active you were before etc (just prostate treatment or nodes as well etc).
most prostate treatment courses are now 4 weeks total and there are a number of trials currently running that complete it within 1 week.
Brachytherapy is another for of radiation therapy where the radiation source is placed in your prostate, typically this can be two treatments or one used in conjunction with a shortened course of external radiation. Ive had a couple of cyclists have that …they were off the bike for a few weeks (the radiation source is inserted through the perineum…anecdotal note; cyclists have quite a tough perineum to get through with needles)