We all probably know about pacing by heart rate. Theoretically, if you keep your exertion light enough that you never cross over into anaerobic respiration, you should be able to continue going at that pace indefinitely (assuming that you aren't doing significant cognitive work - brain exertion doesn't seem to affect HR much). Of course, that isn't how it works in reality. We still need to take breaks from exertion while pacing. What other factors contribute to fatigue?