Last week I was in the mountains camping, and after a 4-5 mi hike up a peak, I decided to run. We were near Grays Peak and there was a bumpy, gravel road from the trailhead that I started following down the mountain. .75mi or so later, I turned, knowing it would be hard to get back.
It was crazy hard.
I had .25mi to go, ran .3, or really slowly jogged, and barely made it. My chest was on fire and I struggled quite a bit to make it that .3 mi. The rest of the way I alternated some walking with some jogging, but it was hard. Definitely a difference running at 12,000 feet.