What causes bonking? Well your body supplies energy to the muscles by metabolizing fats and carbohydrates. Carbohydrates are metabolized much easier and quicker than fats, so that's what the body uses first. Carbohydrates are stored as glycogen in the liver and in the muscles. So what's the problem? Well, the amount of glycogen that the body can store is limited. Actually it can store up to approximately 2000 calories. Guess what? That's enough calories to provide energy for about 20 miles of running. Once this supply is depleted, the body starts to burn fat. Sounds like a good backup plan, but the problem is that the body takes longer to metabolize fat for energy.
What does this mean? Your body begins to slow down because it's running out of fuel. Both your brain and your muscles use glycogen for energy. Your brain is the master control center of the body, so when the glycogen supply starts to run low, the brain gets the remaining glycogen and the legs are left to fend for themselves. Your legs become fatigued and you begin to slow down. When your levels of glycogen plummet even more, your brain starts to become fatigued. Runners often describe this stage like being in a fog. At this point, some runners begin to experience confusion, become very emotional, and some even hallucinate. Hitting the wall or bonking is how your body protects itself. Your body needs glycogen to properly run your brain and muscles. When your glycogen stores get dangerously low, your brain takes over making you slow down to protect itself.
So how do you avoid the wall? Well, it sounds obvious, but a good training plan, race management, and good nutrition can help keep you from hitting that wall. All marathon training plans contain long runs. They're crucial to building endurance. More and more research is showing, however, that not only should runners do their weekly long runs, but they also need to have some at-goal-pace long runs. At some point the body needs to be conditioned to running the longer distances while maintaining the desired goal pace. Also, avoiding getting caught up in the thrill of the race start will also help keep the dreaded wall at bay. In other words, don't start out too fast. Starting out too fast is going to burn up some of that highly prized glycogen you're going to need later in the race to avoid hitting that wall.
Nutrition also plays a key role in avoiding the wall. If you're on a no-carb or low-carb diet you might as well hang up your running shoes. Complex carbohydrates are fuel for the runner. Because your muscles and liver can store only a certain amount of glycogen and because you're constantly using up those reserves in your training, you need to be consuming a steady stream of good, high-quality, complex carbs. Carb-loading the week before a marathon is a common strategy for runners, but runners need to be fueling and refueling the body with carbs long before the last week of training.
What? You've done all of that and you still hit the wall? It happens. If you still hit that wall, there are some things you can try to push you through it. First, if you can muster up the strength, try speeding up. Sounds crazy, but you'll actually use different muscle fibers to speed up. These fibers (or fast-twitch muscles) may still have some energy left to propel you forward while at the same time actually letting the exhausted muscles rest.
Mind over matter, the art of distraction, or putting on your game face can also help you avoid or get through the wall. There are two different approaches you can take—inward or outward. A more experienced or elite runner may take the inward approach—focusing on the needs of his body, taking note of how his body is feeling, monitoring his stride, pace, hydration needs, etc. A less experienced runner may take the outward approach by diverting his attention to something other than his body such as mile markers, water stations, other runners, landmarks, etc. (Personally, counting mile markers has the opposite effect on me. Marking a runner to catch up to works better for me.) The outward approach could also involve the runner using imagery—imagining he's chilling on a beautiful beach—to help him disassociate from the race and the fatigue. Putting on a game face is another outward strategy that's good to use in the last miles of the race. Getting mad or angry and determined to catch that runner ahead of you or to cross that finish line ahead can help you override your central nervous system trying to shut you down. May only last a little while, but it could help you gain a mile or so. It's best to keep the outward approach for later in the race. If you're not aware of your pace or your body's needs earlier in the race, you could find yourself hitting that wall even earlier than expected.
Another strategy to help avoid hitting the wall is to consume carbohydrates while running. This can be done by drinking sports drinks like Gatorade and/or eating carbohydrate gels, jelly beans, or shots such as GU, Power Gel, Accel Gel, Jelly Belly's Sports Beans , GU Chomps, etc. The trick in using these products is ingesting them before you begin to feel fatigued. It takes some time for the carbs to get into your system. You need to be supplying the carbs ahead of the onset of fatigue so when your glycogen stores run low, there's more on the way to replace them. Golden rule of running...never try something during a race that you've never tried in training. So, find out ahead of time which sports drink and sports gel will be provided along the course. If it's not the brand you're using, you'll need to pack your own.
The best depiction of hitting the wall I've seen is in the hilarious movie Run Fat Boy Run. With only three weeks to train, Dennis, the main character, decides to run a marathon to prove to his former fiancé that he can follow-through with something in hopes of winning her back. I won't spoil the scene; you'll have to rent the DVD to see what happens. It's humbling, humorous, and right on the money in its depiction.