The masks have totally been been debunked, so that's an easy pass
What folks experience on a hunt in the West is partly altitude, but a lot of it is simply moving with weight up steep slopes- you can live at 8000' and if you haven't been training with weight moving up a steep slope, guess what- you're going to be hurting, a bunch!
Find something steep, load up a pack (light at first, slowly build) and start hiking hills- that's part of it
The other parts are building a good aerobic base, this takes time (think years, not weeks)- the good news on this front is it's relatively easy to do- 3-4 miles at a moderate pace (hiking or running), moderate being you can still carry on a conversation
Strength train a couple of times a week- concentrate on compound movements- squats, deadlifts, bench, overhead press
Lastly there has been a lot of recent research showing the benefits of training in the heat, not to where you're going to tip over, but moderately. Your body makes physiological changes when exposed to heat, more creation of capillaries in the muscle which equates to more oxygen reaching needy muscles