I think it’s actually mostly a physical problem. When you eat, you store some excess energy in the form of glycogen in the liver and in your muscles. In endurance sports, where you’re exerting yourself for long periods of time, these energy stores will eventually start to get used, and when they get used up, you ‘hit the wall’. This basically means that you’re running out of a major source of energy, and so very suddenly become tired. If you have a strong mindset, you might be able to push on a little bit longer, but the fact remains that you don’t have much energy and will need to replace that to carry on.