Hi Ian,
sorry to hear about your diagnosis, but be assured that there is a great deal of knowledge and support here. It also sounds like you've got a good treatment plan in place.
To your question, back in 2019 I had 16 sessions of radiotherapy targeting my prostate over a period of 22 days. I did start to feel a little tired / fatigued towards the end of the 22 days, but it was quite mild, similar to how you'd feel if you had a common cold. It certainly didn't get to the point that it impaired driving ability and I was quite comfortable driving the 45 minutes to and from the Christie in Manchester for my sessions.
The mild fatigue continued for perhaps a week or 10 days after the sessions finished (the prostate carries on 'cooking' for quite a while after the RT is finished) so I just took things easy and didn't over-exert myself.
Of course, treatments have different side effects on different people. I was 55 at the time I had my RT and relatively fit and active, which may explain why I only experienced mild fatigue. I would advise just to go with how you feel, and if you do start to feel that driving may be a little too much, organise a lift.
I hope this helps, and best of luck with your treatment over the coming months.