You can lose about 7% of your bodyweight in a single day before it starts to impair your performance. For a 180 lb person, that means you can lose almost 13 lbs of sweat. The average human stores about a day's worth of calories in muscle glycogen. Once you burn through that, you'll experience something called "hitting the wall". People who aren't trained for this will quite literally just... stop working. They'll fall over and not be able to move. With training, you can make your body better at burning fat to keep your muscles moving even when you surpass the limits of normal human endurance.
Source: used to run ultramarathons and do alpine style mountaineering