Everyone is pretty much right on the subject here. Hitting the wall and bonking have to do with what your muscles have stored for energy. Heat problems have to do with not enough fluids and not being acclimated to the heat. The more you are subjected to the summer heat and working/exercising in it, the more your body can tolerate heat. I work outside all summer and can handle and perform up to the danger level of heat and humidity and still function at the end of the day. It's the cold weather that I can't seem to handle very long.