I suppose Denver is not so high that lowlanders like us would really have to adapt to widely different conditions. I think Denver is about a mile above sea-level. This is of course something, but there are mountains popular for occasional skying holidays which are of comparable height. They wouldn't be that popular if that altitude was enough to send people panting and gasping. I think the towns in the Andes where people get such problems are considerably higher.
One of my professors once told that he almost collapsed when he left a plane which had landed in Bolivia's capital La Paz. That airport is at a height of 4058 meters.