If the race lasts for 60-200 minutes, take five to six swallows of a 6-8 per cent carbohydrate sports DRINK every 15 minutes during the race
-Peak Performance Online
If you are counting grams or calories, the recommendation is 30 grams of carbohydrate per hour, or a little more than 200 calories.
For races lasting 3-3.5 hours they recommend adding 40 grams of medium chain triglyceride (MCT) to each liter of sports drink and sipping that. I've never shopped for MCT so have no idea if it's readily available. Unless you have access to your own drinks at aid stations, it doesn't make sense to use it during workouts.