Why Don't Sled Dogs Ever Get Tired?

Humans are not the fastest mammals, but can outrun most over long distances, except sled dogs. Sled dogs, especially husky-malamute mixes, excel in ultra marathons due to their unique metabolism. They convert food into energy using glycogen and fats, but unlike humans, they can burn fat and protein without glycogen, allowing them to sustain energy over long races. This ability prevents them from 'bonking' and enables quick recovery, unlike human ultra athletes.

What happens to human runners when they run long distances without sufficient glycogen?

In what way do sled dogs' bodies adapt during a race to avoid running out of glycogen?