Hitting the Wall or Bonking occurs during endurance sports such as running or cycling. It’s when your glycogen (stored energy) within your liver and muscles become depleted causing fatigue and loss of energy. Your body begins to slow down and you can’t force yourself to go any faster, further, or even at all. Brief periods of rest will not be enough to get you back up again.
To avoid “Hitting the Wall” make sure your glycogen levels are high before you run (this is not right before you run but starting days or even weeks before a run/depending on the distance) by consuming carbohydrate-rich foods and drinks. You can consume carb calories during a run with gels to replenish but don’t rely on these only. A typical individual is able to store about 380g of glycogen or 1500kcal. Intense running can easily consume ~600 or more kcal per hour and unless glycogen stores are replenished during exercise, glycogen stores can become depleted after less than 2 hours. To make sure you have an uninterrupted session, training and carb loading can help elevate these reserves.
How do you know you’re body has enough carbs to last you a run?
VO2max, also known as aerobic capacity, is a measure of how much oxygen the body can transport to the muscles and consume during aerobic exercises. Those who have higher VO2max values can last longer and exercise harder. To find an estimate of your VO2max divide your maximum heart rate by your resting heart rate then multiply it by 15. To find your maximum heart rate simply subtract your age from 220 beats per minute. The result will state how many excess carb calories you would need to intake before a race. ~ Reuters
You can also avoid “Hitting the Wall” by progressing your weekly long run each week, allowing your body’s capacity to store more glycogen. By increasing your glycogen stores you are able to push more and learn to fight fatigue as well as teaching your body to utilize energy reserves from fat storage sites after the glycogen stores have been used up. Try training with your desired marathon finish time in mind. Have a goal when you run and focus on that goal during training. You don’t want to run your entire training run at marathon pace but do try to push it near the end of your run. Your body will become more efficient at running your goal pace even when already fatigued. Don’t start off too fast or you will burn through your glycogen stores too early.
Even though I am sharing this all with you I'll admit that I personally have a hard time with this as I am not a big carb consumer at all, so I supplement, but supplementing can be very expensive. I am making more of an effort to consume more carbohydrate-rich substances. Too many times I've bonked way too early and it's dang frustrating. Days I've carbed up, I've noted to go harder longer. The next time you all of a sudden feel like your legs are like led, as though your body has shut down and leaves you feeling like a quitter, carb up!