Starfish - I live virtually at sea level (about 1000 ft. elevation here at my house), but go to the Rockies every summer to ride. The only thing I would add to what everyone else has said is to really pay attention to heartrate. I don't rely so much on percentage as I do the actual number. Through the years I have learned that I can ride at altitude (7,000 ft. and over) if I keep my heartrate below a certain number. Above that and I crack because of lack of oxygen. I am not and never will be a fast climber, slow and steady is more my style, but eventually I make it to the top.