its not something that is going to make you sick, or at least not likely unless you've never been in elevation, then if you go too high, its definitely possible... trust me the very first time i went extremely high in the mountains, i threw up the entire next day but it was an extremely high elevation and i was very young... when it comes to training, colorado is one of the best places to get into shape but trust me when i tell you, i don't care what anyone says, ive experienced it and have many clients that have the same, when you are in that altitude, it is FAR more difficult to do endurance training HOWEVER when you get to the level where your able to get past the initial difficulty, you have a huge advantage on vo2max and cardiovascular output by training in that kind of elevation... its night and day... so many professional endurance athletes move to colorado to specifically prepare for events because of the difficulty of training and what you can get out of it...
