Raimius is absolutely correct re: fitness and the focus on cardio! In fact, he hits all the points precisely: setup a great workout plan with the excercises he lists...and a lot of core work: planks, etc., and then run...don't go for speed, go for endurance. If you can't go out and "liesurely" run 3-4 miles, whenever asked, then it's time to start building up.
Altitude...you can't do anything about that unless you move to a high elevation. This is a physiological issue...to avoid the long classroom sounding explanation, let's put it this way: your body will produce a LOT more red blood cells when you move up in elevation to acclimate you to the altitude. The process takes @23 days, give or take per person....so by the time you hit Jack's Valley, you're pretty close to "normal."
Steve
USAFA ALO
USAFA '83