i'd take a page from the paleo handbook on this one. For a million years, homo sapiens were a nomadic people and did a lot of walking on nothing more than nuts, raw meat, insects and foraged greens.

A trail mix that you make yourself, without the dried fruit, would be more carby than perhaps you'd normally eat, but you'd burn off the carbs and they probably wouldn't be your own "trigger carbs" so that you'd come home craving...a variety of nuts, sunflower and pumpkin seeds, real fresh coconut. All these are also calorically dense, so for little weight you can fuel your work! Also, the recipes for homemade protein bars that include peanut butter and protein powder sound portable and fun. Since you will be burning off the carbs, a dense fruit like an apple would probably not be a problem either. HFS or homemade jerky is another possibility.

I'm impressed with what you're doing!--I love walking but never in my life tried more than 13 miles! You GO!