I am a 4:37 miler (I think I can go much faster, but I digress) and just ran a 6:20 mile at 12,000 feet. It was on my second attempt and my first, I made it about 0.2 miles at 4:27 pace before I had to stop. Granted 6:20 was maybe a little conservative and maybe I could have pushed for 5:45 (close to sea-level tempo pace) but it was NOT easy!
Not sure if anyone has developed a conversion for 12,000 ft, but I am going to go out in a limb and say that it takes a minute at least off your mile...