Used to ferry planes, one in particular, think a cessna 208 or 182 or something, from Fresno to Palomar. I flew the route all the way at 12000ft to get the quickest route over the San Gabriel mountains and LAX. wonderful view, no probs. Also a good friend flys a taxi and when empty or freighting it regularly goes to 14000ft over alps with no problems, and he smokes. So if you are reasonably fit and health, it can be ok, but perhaps better not to try, as it could be a bad way to discover you are not as fit as you thought.