You should have plenty of time to recover from SBRT. It usually causes some urinary issues, Taking Flomax can almost always resolve those problem in a short time

Are you having a barrier like SpaceOAR, Barrigel, or BioProtect put in so that it can protect your rectum.

My brother had almost identical procedure to yours at 77, After three months, he was doing just fine. He was getting some hot flashes, but it wasn’t debilitating.

Yes, it’s true that some people can have some pretty serious side effects but it is a very small percentage. If you’re in good enough shape that you can do that kind of hike now then doing it in September shouldn’t be a problem.