Physical exertion of any type at high altitudes, or anything between 6,000 to 8,000 feet, brings with it a set of complications you won't find when training at sea level or close to sea level. The biggest problem is in the air. The higher the altitude, the lower the oxygen content of the air you breath and the harder it is for those of us who live at lower altitudes to get enough oxygen to our bloodstreams. As a result, if you aren't used to high altitudes, you will experience shortness of breath much faster than you would at your normal altitude. In fact, if you don't carefully acclimate, you can wind up with potentially serious or even life-threatening illnesses.
The first thing to do if you plan to train at high altitude is to take it slow. This may mean arriving a few days earlier and letting your body acclimate before you actually start training. Some easy walks, increasing distance and speed slowly, will help you get used to the rarified atmosphere. It takes about three days to acclimate, so don't plan to go any higher until you take time to acclimate at your current altitude. At an altitude of 6500 feet, you can lose between 10 and 12 percent of your VO2 max. This increases to 12 to 15 percent at an altitude of 7500. The higher the altitude, the less oxygen you will have available until your acclimate.
Plan to get to your high-altitude destination with enough time to catch a good night's sleep before you start to train. Sleeping is a good way to let your body begin the acclimation process. Even after a few days to acclimate, start your training runs slow and easy. Give yourself plenty of time to work your way back to the hard training you may be used to. If you aren't used to training at high altitude, make sure you add in as many days as possible to get your body used to the altitude before you have to race.
Make sure that you stay hydrated. You dehydrate more quickly at high altitudes, so make sure you drink plenty of water before, during, and after your training. Even if you are running in the snow, don't count on snow to provide the hydration you need. Your body has to work overtime to melt the snow, and the coolness of the water produced lowers your core body temperature. Instead, carry extra water with you or use a hydration backpack. Don't use alcohol, tobacco, or any depressant drugs while at altitude.
Sunscreen becomes even more of a necessity when training outdoors at altitude. The higher the altitude, the fewer atmospheric barriers you have between yourself and the dangerous rays that cause sunburn and cancer. It may not seem like much, but you'll definitely feel the burn if you forget to keep your sunscreen applied.
Altitude sickness is no respecter of fitness level. Even the fittest person can find herself felled by a serious bout of altitude sickness. If this is your first trip to high altitude, or if you are going to an altitude considerably higher than you've been before, talk to your doctor about a good altitude sickness medication. If you know you have altitude sickness, take the first dose while you are still in the air so it has time to work before the cabin of your plane is depressurized. Then take as directed to make sure you don't get sick.