I didn't have too much trouble with altitude on my US trips, which all started from Denver, except when I went up into Rocky Mountain National Park from Boulder and passed the 10,000' mark.
Distinct shortness of breath was evident. (We're about 450' asl here).

The real problem was the upheaval experienced by my internal clock.
On my first night, on one trip, I was wide awake at 0200 with my stomach telling me that it was 0900 and where the hell was breakfast?
As a diversion, I switched on the TV to fill in an hour or two and watched an episode of Dad's Army in the middle of the night on Public TV. :)

That sounds.... filling.