I’ve been very good about exercising here in Denver.  I swam laps one day, and have hit the treadmill for the past two days (granted I’ve been eating out every night, so it’s probably a wash, but that’s another story).  The 45 minutes of swimming took a lot out of me, but I wasn’t too surprised about that — I’m still at the very beginning of building up any sort of endurance — but my first day on the treadmill, I could barely handle the interval workout I normally do at home.  I ordinarily spend 45 minutes on the treadmill, adjusting the speed and incline quite a bit, and my workout includes several rounds of sprints alternated with jogging to recover.  After my first sprint on the hotel treadmill, I had to slow down to a walk to recover.  I couldn’t figure out what was going on — maybe I was dehydrated, maybe I was tired, or maybe it was the second martini I had had with dinner the night before?  Then, over dinner last night, someone mentioned the altitude, and how she had been warned to stay hydrated.  Another mentioned that she was told not to exercise at all because she might have trouble with the altitude.  Aha!  Mystery solved.