Yeah, altitude related....Our camp was at 11,200 and we dabbled around 12 ,000 quite a bit.
My cousin coming from California (teacher-38), had to go until 3 PM at his school before he could get on the road. We were told to be ready to go at 9 AM (though due to the Alabama idiots, this would not be an issue anyway). So he drove straight through from LA, and rolled in 8:30 and had his stuff ready to go by 8:45. Then it was hurry up and wait. At 11:30 we packed up, and headed to our camp. So he went from LA to 11,000+ in less than 24 hours. He did fine that day, had the standard breathing fun opening day, but he is a former Marine and we saw quite a few assorted elk opening morning, so he powered through well enough. 2nd morning he woke up feeling like total "dog sh--". He stayed back at camp, took a couple Advil and jugged water like a mad man. By mid afternoon he was able to hunt some around camp some.
I left DFW, and drove to Denver to say with a 3rd cousin the first night (for me), and then picked up Pennsylvania cousin at the Denver airport and then headed up to Trapper Lake Lodge (9,600 ft). And we took our good sweet time sightseeing from Denver to Trapper Lake/Meeker area (aka buying our supplies). He and I were making a point to drink water like crazy, and had some minor acclamation. We met AgLa06 and his buddy at the Lodge.
My Penn cousin is a former college baseball player, that was 32.....ie fairly big dude. He had the worst cardio, but was a out as strong as me and Cali cousin combined. The evening I shot my elk (the 25th), it physically beat the hell out of us. Starting at basically dark, we took it all the way to boning it out. And hauled 4 out of 6 full game bags back to camp, plus the head. That was honestly one of the most physically challenging things I have ever done.
They next morning, we were hurting, but headed back to get the other 2 bags. Out of the blue, Penn cousin got a weird second wind, and damn near ran off with the larger of the 2 bags and I'm not sure if he stopped before getting back to camp. Me and marine were joking he'd feel that later. "Man, I don't know where that came from, but I feel great". Well, starting around dinner time he started getting some leg cramping. Next morning he had all sorts of cramping, and headaches. Water drinking and he was knocked out for the day. By dinner he was back to normal, but effectively missed a day.
Biggest difference was I was the oldest, and had done my darnest to make sure I wasn't playing with heart attack in the mountains. I had worked my a-- off on cardio. I've also never been inclined to have altitude issues beside the initial fun of breathing. I would get tired for sure, but even to my surprise I recovered pretty fast......and all that work has pretty well gone out the door, amazing what you can do with a major end goal.