I lived at 600 feet in IL before moving back here, and I really couldn't tell the difference. When I moved here the first time, 21 years ago, it took me about a day to adjust, but I never really had a problem with it. Other people suffer bad when moving here from a low elevation. I couldn't tell any difference when we were at our house in Idaho, compared to here, and I sure didn't have more energy! We were there for 2 weeks, and I could have used a burst of energy! Many people are fine here once they get used to it, but if they leave for any time, like for a vacation, they have to readjust to it when they get back home. I'm just lucky, I guess.
About the only time I've noticed thinner air here is when my husband and I used to drive to the mountains a couple of times a week to look at the wildflowers and wildlife. We have a GPS in our truck, and the elevation was 13,000 feet where we'd always park and enjoy the view. The first time we went up there, I got out of the truck and hiked up a steep road a bit, to see if we could easily drive up it, and I got winded kind of quickly, which surprised me, until I realized how high up we were! Then I didn't feel so bad!
The area in Idaho that we're moving to is supposed to have the 2nd longest growing season in the state, with the 1st longest being Lewiston, which is about 30 miles west of there. I'm so excited about having a longer growing season!