Unlike “regular” dogs, sled dogs like Siberian Huskies are specifically designed to be incredibly athletic and to be able to run for so long without negative consequence. They have larger hearts to pump more blood more quickly, and through selective breeding, they’ve adapted to their functions (running great distances while pulling a heavy sled, for instance) by developing a sort of metabolic on/off switch. You know how humans produce energy?
Basically, they burn glucose, which is a simple sugar. When they run out of that, they start to burn fat. Fat requires much more oxygen to burn than does glucose, so it’s a lot more taxing on the system. As a result, fatigue sets in. Sled dogs (who typically consume very fatty diets) burn glucose at rest and in the beginning of long exercise, but after awhile, they’re able to switch to burning fat instead. They’re also able to pull as much as twenty times as much energy from their bloodstream as humans. It allows them to exercise hard for insane amounts of time without succumbing to fatigue. The exact mechanism that allows them to use their metabolic on/off switch is not totally understood. Scientists are working hard to figure it out, since they believe that it may benefit humans.