I'm a flatlander. Several years back, I had business in Denver but wasn't sure when it would conclude. Turns out I finished on Friday but wasn't due to fly back until Monday so I rented a car to explore for the weekend. Ended up Friday night in Blackhawk (elev about 8500). A rain made me have to sprint about 30 yards to my car, thought I was going to expire. On Saturday, I happened on Mt. Evans, the highest automobile accessible road in the country at somewhere around 15,000 feet. My poor little 4 cyl rental was wheezing at full throttle in first gear on the insanely steep switchbacks as I neared the summit. I was in worst shape than the car, I parked and walked the final few hundred feet up a slight grade to an observation platform feeling like there was no air at all. As I walked down back to my car certain that human life wasn't possible at that altitude, two bicyclers arrived from the access road into the parking area. This was years before I started biking but I was impressed, still am.

A lady in Blackhawk who wintered in California said that, on return to altitude, it took her a couple of weeks to become acclimated.