I had a brief "altitude camp" experience when I was in my 20's. I went to my brother's college graduation in Santa Fe, as I remember I was there for 3 days, went running a couple of times.
Came back, went on a run (wasn't cycling at the time) and I was figuratively blasting up hills that I'd chugged up the week before. And this was after only 3 days at altitude. Unfortunately the effect didn't last for long, but man, I still remember that run even though it was more than 30 years ago now.