So I've done some research to figure out what went wrong during the 1/2 marathon.
"Bonking" seems to mean different things to different people, but what I clearly suffered from is a depletion of glycogen stores.
During exercise, your body uses two sources of fuel: glycogen and fat.
Fat: If you're like me, you have no shortage of fat! Many many thousands of calories of fat. But while fat contributes energy to your effort, it can't sustain intensity.
Glycogen: This is the gunpowder, the stuff your body needs to sustain an intense effort. Your body can store around 2000 calories this way, if you've done everything right to maximize your stores.
You burn the same number of calories running no matter what the intensity (more or less). But what changes is the type of calorie you're burning.
At low intensity, you burn more fat per kilometer. At high intensity, you burn more glycogen.
So despite my being able to do a 24k run a few weeks ago, in the 21.1K half marathon I was bagged by 15K. This was clearly down to the intensity. 24K @ 6:00/km relies less on glycogen than 21.1K @ 5:24/km.
1) Carbo-load. I thought I had done this, I ate a lot of complex carbs the day before the race. Whole wheat crackers, whole wheat spaghetti, etc. In the morning I woke up and had Mini Wheat cereal, followed by multigrain toast and a little bit of egg, and some milk, and Coke... right before the run I ate a banana and was drinking Gatorade.
But the race was right at my normal lunch time, and I think I may have (unbelievably) under-ate.
2) Eat during the run. I thought the Gatorade would be sufficient, but it wasn't. I splashed more on my shirt than I got into my mouth most times - I need to practice drinking from a cup while running! I really should have gone with some gels, I forgot to buy some and was unprepared. Next time I'll target 250 calories an hour during the run.
3) Lose weight. I'm a big guy, around 190lbs, and carrying all that weight around 21.1k takes energy. Using the formula :
Calories burned = 0.4785 * weight in pounds * kilometers run
I discover that each pound of weight takes 10 calories to move 21.1K. Dropping 10 pounds would save 200 badly needed calories over a full marathon... mostly on the glycogen side.
4) Slow down. I'm not sure this is an option, because other than this bonking thing I felt the pace was good. If I had the energy I should have been able to go faster, it was in me, just ran out of gas.
What really burns me is that I knew about the importance of all of this... I did a century ride on my bike, over 5 hours of riding. Fueling up and staying fueled up was critical! I just didn't think 2 hours of running would be so hard, but I underestimated intensity.