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20000 calories? That must be american calories. If we subtract 2000 kcal for general upkeep that's still the equivalent of pedaling on an ergo at 220 watts for a continuous 24 hours. Half the population can't sustain that for a mere 20 minutes.

Why can't they? Because it exceeds their maximum rate of oxygen consumption. That also tells us they can't in any scenario burn that many calories in a day.

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Most of the calories spent not on muscle movement that consumes oxygen, but on warming up the body. That probably also consumes oxygen, but I'm not sure how much.

When working, keeping warm doesn't use additional calories to the work (skiing and sled-pulling), since heat is a biproduct of the work. In fact they are careful not to work too hard so they don't sweat. (Getting wet makes their insulation ineffective, so is very dangerous.)

Generating energy takes roughly the same amount of oxygen either way regardless of whether you're pedaling a bike or just staying warm in a freezing environment.

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