As a competitive athlete for the past 40 years plus the riding, the optimal fuel mix for athletes as been (40 % (carbs), 30% (pro) and 30% (fat). The study was done on world class aerobic swimmers.
Ah, Barry Sears, PhD: The Zone. Fascinating stuff. Tweak that to ready 40% low glycemic carbs; 30% low fat, high quality proteins; and 30% good-for-you fats. A Zone nutrition plan (not a diet) confronts the participant (not the dieter) with mountains of delicious and wholesome foods. Hard to get it all down but there's a wide variety of superb meals available easily assembled form a huge list of acceptable and readily available ingredients.
Much of Sears' research and findings are being repackaged these days. Science and biology, of course, have marched on and there are new theories all the time. Pick a guru and test your nutrition plan for several months before you attempt to ride across the country. Stay flexible. Be adaptable. Try to make foods that allow you to enjoy what you must eat.