As always, YMMV. I can run a marathon in under 3:00, and making an airport connection in Denver probably wouldn't phase me much (I've run in Denver, but not to make a flight). OTOH, I distinctly remember going up Pike's Peak at >14K feet, and found the walk across the parking lot to be challenging. I can't imagine what climbing a >20K foot peak must be like given that humbling experience.