I've read that it takes one day for every 1000ft in elevation change for the body to effectively adjust, and I've found this to be pretty close to true. As everyone has stated, staying hydrated is key to feeling good during the hunt. Drink, even when you're not thirsty, it certainly helps. I've found that flying out several days ahead of time and even relaxing at a hotelfor a day or two helps before heading into camp.