John Basedow: No training can replicate physical activity at a high altitude, which means many athletes succumb to altitude sickness, when exercising in elevated locations.
Here are my tips for athletes that need to acclimate to high altitude activities. No what shape you're in an altitude over 6000 feet lacks the same amount of oxygen your body is used to, which can cause acute mountain sickness or AMS, symptoms include headaches, nausea, insomnia, shortness of breath and grogginess. It's easier to prevent these symptoms than treat them, so have a plane.
If you're competing in a high altitude race, game or athletic activity get your location a few days early to get used to the air, if you're on vacation take your day or two to adjust before you exert yourself. I'm an advocate of slowdown but never stop. Base your exercise on the effort you expend not how long you're active, pretend like you're starting from scratch and complete as much as you can before you feel fatigued. Dehydration increases your risk of sickness so stay away from coffee and other caffeinated products and drink plenty of water. If you're prone to AMS or don't have time to acclimate ask your doctor for a prescription for Acetazolamide, this medication accelerates the body's ability to adjust a low oxygen level and treat symptoms after they hit. Descend to a lower altitude, drink plenty of fluids, take medicine for a headache and get access to oxygen if possible.
When you prepare for thin air you'll be ready to perform at your peak.