I drove for 23 sessions, 40 mins each way. I didn't have any problems.
I did wait around for 30-60 mins after each session to allow a couple of pees before returning, either talking with other patients or going to the Macmillan Cancer lounge and doing some work on the laptop.
A few people do get seriously fatigued during radiotherapy. I think doing exercise and being fit is probably reduces that effect - I didn't get fatigued during the external beam radiotherapy, and often went out for a bicycle ride later in the day.
In case you aren't yet, do start doing pelvic floor exercises. They may help you stay continent in the latter stages of the treatment.