Same. I've climbed Mount Adams (12,280 feet) several times, including once with an overnight stay at 9,500 feet as well as other times when I did the whole ascent and descent in a single day.

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.