I was right - ish :)
I found this which explains it - "In endurance sports such as cycling and running, hitting the wall or the bonk describes a condition caused by the depletion of glycogen stores in the liver and muscles, which manifests itself by sudden fatigue and loss of energy. Milder instances can be remedied by brief rest and the ingestion of food or drinks containing carbohydrates. The condition can usually be avoided by ensuring that glycogen levels are high when the exercise begins, maintaining glucose levels during exercise by eating or drinking carbohydrate-rich substances, or by reducing exercise intensity."
"Athletes engaged in exercise over a long period of time produce energy via two mechanisms, both facilitated by oxygen:
via fat metabolism and
via breakdown of glycogen into glucose, followed by glycolysis.
How much energy comes from either source depends on the intensity of the exercise. During intense exercise that approaches one's VO2 max, most of the energy comes from glycogen.
A typical untrained individual on an average diet is able to store about 380 grams of glycogen, or 1500 kcal, in the body, though much of that amount is spread throughout the muscular system and may not be available for any specific type of exercise. Intense cycling or running can easily consume 600-800 or more kcal per hour. Unless glycogen stores are replenished during exercise, glycogen stores in such an individual will be depleted after less than 2 hours of continuous cycling or 15 miles (24 km) of running. Training and carbohydrate loading can raise these reserves as high as 880 g (3600 kcal), correspondingly raising the potential for uninterrupted exercise."
Taking gels or sports beans when I feel that dip definitely helps. Which reminds me, I need to pop to Sweatshop for some more gels and electrolytes today.