Long time elk hunter (Wyo) here but living in the flatlands (Okla).
You have an advantage living at 5000 ft in Denver so your acclimating is probably quicker.
I'm almost 71yo but stay active and strength train year round. My first day in the mountains is slow but that isn't a bad thing when hunting.
I focus more on my strength training (HLM) and daily hiking outside with regular daypack weights to make sure my boots are comfortable.
I do hard cycling intervals on a 30-60 second hill once or twice a week (30 minutes tops).
I do not run. I've never needed to run to kill an elk as I know they are always faster.
I don't spend a lot of time with very heavy rucking because it affects my recovery for strength training and my archery practice (2 Factor Model?). I may load up a heavy pack just once to make sure all equipment is in order. My strength training alone is enough for me to pack an elk out. Sure, it's going to suck and I always use trekking poles but it's not an impossible task and there is adrenaline after being successful. Plus, you pack out the elk and the heavy lifting is done except for helping your buddy pack his elk out.
Good luck this season. September is getting close.