I was Air Force and we only had a 1.5 mile run to do (at least when I was in). I only ever had 1 time I failed. I had been stationed in England for 2 years and changed duty stations to NORAD in the winter. I went from sea level to over 6,000 feet above sea level. I had to do my run the 1st week I was there, and it had to be a bunch of laps around inside the gym because everything else was covered in snow. My lungs felt like they were going to burst and I failed the test. The guy that gave the test told me not to worry - I would get to do it in another 30 days. He said most people had a hard time passing it their first time - especially coming from sea level. They gave people a chance to see if they would pass it or not right away. I think it was to laugh at us trying to breathe.

Once I acclimated I passed the run easily (and it was outside by then).