Interesting piece of research here that found a correlation between finish times in an Ironman Triathlon and intake of carbohydrate and water.
"the correlation of finish time with the rate of carbohydrate intake relative to body mass could be from -0.74 to -0.39, which represent improvements in performance of between 7.2% and 3.6% for an increase in carbohydrate intake of 0.40 g·kg-1·h-1 (a change of 1.0 standard deviation). The most likely gain in performance is about 5%, or about half an hour over 10 hours."
The authors concluded "Ironman triathletes who increase their below-average consumption of food and drinks in an event can expect to enhance performance by at least a few percent. These findings should apply to a substantial proportion of athletes in all ultra-endurance events lasting 8-15 h."
In other words, get your calories- approx. 1 gram per minute of carbohydrate, or 240 calories per hour.