The experience was very similar to attempting to run at 7200 feet having been acclimated to sea level.
I used to bike to work a few times a week, an easy five mile ride to my old office. One year after giving blood (not the same day but definitely the same week), the ride was awful. It was almost painful. I'm at about 900-1000' above sea level here, and that memory is surprisingly similar to how I felt hiking at 10,000' after living my whole life at 1000'.