Even if you don't feel it, the altitude still makes a difference, though. I recall doing two-a-day hell weeks at Big Bear at the end of summer cross-country training in high school and there was a 5k up there at the end of that week. We all got worse times than typical at sea level, and somewhat amusingly, I recall a high school senior from Rim of the World High School (who lived up there) getting 2nd place overall the first year I ever competed in that race, beating way more seasoned competitors just because he was used to the altitude.
It works in reverse, too. There was an officer in my Armor Basic Officer Course from Colorado who gave himself rhabdo during the two-mile test the first week we in-processed, apparently because he was so used to altitude that he hadn't quite acclimated to Fort Knox atmosphere.
Altitude sickness typically starts after 12–24 hours. If you climb high and come back down in the same day, there is usually not enough time for the symptoms to start. And 11,500ft is not that high altitude. People routinely fly to Cuzco, La Paz, Lhasa, and Leh from sea level, and most of them suffer no serious ill effects.
The better your heart is at getting oxygen into your muscles and organs, the better it can compensate for less oxygen.
Not a bulletproof solution to altitude sickness, but it's definitely one of a lot of variables that matters. It's also just true that some people are way more susceptible regardless, I've got friends who run competitive marathon times who get splitting headaches flying from sea level to denver.
Not really. Altitude sickness seems quite random in who it effects worst. I trekked to the top of Mera Peak (~21,000 ft) many years ago. 3 of the fittest people in our party got altitude sickness and didn't make it to the peak.
Other amusing things from that trip: we went up there the 3rd of July, and it snowed. We charged the car in Colorado Springs before we left, got up to the peak with 36% battery remaining. My wife worried we wouldn't be able to make it back. Got back to CS with ~70% battery left.
It's a tiring climb and a tiring descent, but I never felt a hint of altitude-related discomfort.
I lived near sea level and didn't often go anywhere more than about 1,000 feet above sea level in daily activities.
