Carbohydrates are a short explosive energy source, that are of high volume (one molecule of carb holds seven of water) and limited amount in the body. You can carb load, overcompensate, whatever you want, but you can not go above a certain level.
Fat is a dense energy source that can keep you going further and for more time, obviously at a lower pace. I did not read in depth sledge dogs studies but I have read human on hypermarathon athletes, you know, the guys that run 100+ miles per day for a week etc. Even those subjects , that consume carbs during the race at frequent intervals, it is the fat burning they rely on.