Your brain gets almost all of its fuel from sugar in your bloodstream. When your blood sugar level drops, your brain cannot get enough fuel to function properly, you feel tired and confused and can pass out. There is only enough sugar in your bloodstream to last three minutes. To keep your blood sugar level from dropping, your liver must constantly release sugar from its cells into your bloodstream, but there is only enough sugar in your liver to last 12 hours at rest. During intense exercise, your muscles draw sugar from your bloodstream at a rapid rate. Your liver can run out of its stored sugar and your blood sugar level can drop, and you bonk.
Bonking is common in bicycle races if a rider does not eat frequently, but is rare in long distance running races. When you run, your leg muscles are damaged from the constant pounding on the roads and you must slow down. However, you pedal in a smooth rotary motion which does not damage your muscles, so you can continue to pedal at a rapid cadence for many hours.
To prevent your blood sugar from dropping too low during intense exercise lasting more than two hours, eat at least every 15 minutes. It doesn't matter what you eat: salted peanuts, a peanut butter and jelly sandwich, chicken, an apple, a banana or anything else. Almost all fit people can take small amounts of food frequently during exercise without developing stomach cramps.
Source: Dr.Mirkin.com/ photo from cyclejerk