You may have heard of "runner's high." It refers to the release of endorphins in the brain when your body's reached a certain level of physical exertion. And, it's awesome! Interestingly though, this high comes at around the same time that your body's glycogen stores are depleted, putting you at increased risk for injury.
Glycogen is the way your liver and muscles store glucose to use for energy. During intense exercise, like running, you use it all up in under 2 hours, which will cause your body to begin breaking down muscle and fat to restore blood glucose levels. Repeatedly putting your body in this state may increase your risk for sport-related injury. So if you're undergoing a serious training regimen, consider looking into carbohydrate loading. And, like anything, remember to treat your body well, eat right, take days off, and don't get caught being a junkie!