I'm not a nutritionalist (but i did stay at a HI last night...). seriously i WAS a 10-S player in college many years ago & think there may be more parallels between comp. tennis and long distance hikng in mountains (as opposed to marathon running). i think, like competitive 10-S, mt. hiking is not really an aerobic activity so much as sustained rest/recovery semi-anaerobic activty & would also be willing to bet yer bonking not so much from lack of calories (which aren't that hard to replace if timed properly) but more from fluid and in particular electrolyte/mineral loss. of course nothing prepares you like training the same way you will hike, but we always found when we had to do two long matches in a day - that's miles of sprinting and even more miles of walking (think uphill, level, downhill) the only way we could pull it off was to carefully monitor our fluid and electrolyte intake. i.e., have constant intake of a very watered down version of sports drink coupled w/ pieces of banana, raisins, fruit every couple of hours. also helps to keep body temp down in summer if that is factor. but in my experience there's not much you can after you start to feel bad, by then it's too late (candy provides a short sugar energy burst, then you crash). so i would try mixing a SLIGHT amount of gatorade whatever n your water, sip constantly, and snack on fresh fruits along the way. try it!