I was diagnosed 5 years ago at age 54 with inoperable stage IV PC, mets in my pelvis, spine, ribs etc. I was (and still am) fairly fit, though certainly not marathon-running fit like your dad!
I had initial hormone therapy, then 6 cycles of docetaxel (at 3-weekly intervals) followed by 18 cycles of radiotherapy. I've also been taking enzalutamide for the past 3 years.
Docetaxel chemo is mildly unpleasant, but not horrible, and the side-effects follow a predictable cycle. You get steroids before each infusion, which make you feel rather lively, then 48 hours after the infusion the steroid effect wears off and you 'crash.' You feel like you've got a bad cold (lethargic, tired, achy) for 7 days or so, then you start to feel better. Then you're fine for 10 days, and it's time for another infusion. Rinse & repeat. Hair loss is common (mine grew back, it doesn't for some people), as is loss of appetite for a while.
I've found very few side effects from enza, apart from complete loss of libido and the occasional hot flush. Some people have more and worse side-effects.
My 10p worth would be to go hard and go with the chemo as it suppressed my PSA very effectively for 2 years. It did impact my QoL a little but I had my infusions done in April - June and being able to get out in bright, warmer weather helped me deal with the side-effects and improved my frame of mind no end.
As mentioned by another poster, ask if chemo AND enza is possible. Whatever your dad decides, best of luck to him, to you and your family. Don't be afraid to ask questions of the oncologist, and on here to help inform your decisions.