I know that in hockey teams that go in there try to shorten their shifts.
Someone suggested to me that playing at altitude wasn't as ******* football players as it is on NBA/NHL players. The reason, NBA/NHL players are always moving.
I called BS on this notion. Playing football doesn't have the constant, fluid motion of the NBA/NHL, but it is far more intense. I think of the NBA/NHL players as endurance athletes, runners, and NFL players as sprinters. Once the ball is snapped, every player is playing flat-out intense (okay, maybe not place kickers or punters).
(And take a look especially at the OL and DL guys after a series. They're on the sidelines, sweating even on cold days, and breathing hard.)
So 60 minutes of football is like "high-intensity interval training (HIIT)", where you go flat out for a short period of time, and then only get a short rest (40 seconds between plays in the NFL, I think), and then you have to go at it again.
I do HIIT workouts on a stationary bike (spinning), and I alternate 30 seconds all out with 30 seconds of recovery. It doesn't take long to get gassed.
As for the Packers playing in Denver, it is what it is. I think by this time in the season these guys are in very good condition. They will sub more often and make generous use of oxygen on the sidelines.