All of this science if quite interesting, but even more valuable is the example set by the most successful runners. The best runners in the world, as everyone knows, are the Kenyans. The diet of the typical Kenyan runner is 76 percent carbohydrate. Compare that to the diet of the typical American, which is less than 50 percent carbohydrate. Kenyan runners get the majority of their calories from ugali, a dish made entirely from cornmeal, that supplies a whopping 38.5 grams of carbs per half-cup serving. The only runners whose abilities rival the Kenyans are the Ethiopians. The diet of the typical Ethiopian runner is 78 percent carbohydrate!
This does not mean that you should automatically aim to get more than three-quarters of your daily calories from carbs. The amount of carbohydrate a runner needs in order to handle his or her training is tied to the amount of training he or she does. In my book, The New Rules of Marathon and Half-Marathon Nutrition, you will find a handy table that tells you how much carbohydrate to include in your diet based on how much you train (and your weight). The requirements vary from as little as 3 grams of carbs per kilogram of body weight daily for those who do just a few short runs per week all the way up to 10 g/kg for the heaviest trainers.
If you train very lightly, it is possible that you are already consuming more carbohydrate than you need to optimize your training capacity. But it's more likely that the guidelines in that table will require you to adjust your carbohydrate intake upward, especially during periods of heavy training, such as before a marathon. If you do, I am certain that you will get more out of your training and reach the finish line of your event faster.marathon.