How do I avoid exhaustion from physical activity?

So I go airsofting quite a bit (like once every week), and we are normally out there for about 4 hours. The temperature is normally in the high 70s low 80s, and we wear dark, camouflage. At about 2 hours in I get really exhausted, like I just want to fall over and take a nap. I get about 8 hours of sleep, Eat a good breakfast (Today consisting of 2 eggs, 2 pieces of wheat toast with margarine spread, a tall glass of milk, and a small bowl of Cheerios), and I drink a good size glass of water before we leave. However, I don't really get anything to drink while im there save for a few drinks of my friends canteens. Is this dehydration fatigue? Am I not getting enough sleep? Am I not eating enough calories?