I hesitated to respond, but figured I should add my bit as my experience was different from the above.
In 2013 I got 40 radiotherapy sessions (a total of 72 Grey) to the prostate bed and surround 3 months after RARP. I got radiation proctitis, explosive diarrhea + intestinal cramps, an increase in urinary incontinence and a major increase of bleeding from my hemorrhoids. Fatigue was pretty serious for quite some time (my wife and son could not wake me up when I fell asleep for several months after the last treatment). The other side effects steadily increased for 3 years and 8 months after the end of treatment, then they began to subside.
The intensity of the radiation, the precision of the treatment and the tightness of the beam focus have all improved quite a lot since then (10 years now), and the side effects from my most recent radiation treatments (2 years ago) are much less. Contrary to others' experience I had to empty both my colon and my bladder before any and all radiation treatments. Do as your therapist says and it will go well.