You've got a problem you need to figure out before my approach would be worth considering, i.e. you have to figure out how to "bulk up" a bit before you start a hike. I've got pretty much the same problem you mention in my frontcountry life-I am 5'7"/137# and do not gain weight easily. But, I make a concerted effort and generally gain about 4# of body fat in the month before a hike of 10-11 days. It ain't easy for me either, but I just go ahead and eat more (and more often) until I get there. Then I use my backpacking food to provide the carbs to burn my body fat, plus ~45% of my dietary calories as fat to augment the body fat calories, plus ~10%(270 calories) as protein to provide a little protein for energy plus enough to "mostly" repair damaged muscle tissue. My carried food weighs 20-21 oz/day and contains ~27-2800 calories. This plus 4# body fat at 3500 cal/#(14000 calories) yields ~4200 calories/day, which seems to be about what I consume, although possibly a bit less. I sometimes come back a "little skinnier" than when I left(1# or so after I rehydrate for a couple of days), so I am occasionally undershooting a little and using some additional body fat(not enough to damage me as far as I can tell, so far, and I have been doing this for several years now). As for what I carry: A typical breakfast is: 1) Seattle Chocolate bar @ 2.5 oz, 394 cal; 2) Bear Naked High Sierra Blend(a trail mix from Trader Joe's) @ 1 oz, 145 cal; 3) Onion Poppy Seed Sticks(I get them at Whole Foods, probably available elsewhere, too) @ 1 oz, 155 cal; 4) Macadamia nuts @ .5 oz, 97 cal. This adds up to 5 oz, and 792 cal with 57 gr CHO, 15 gr PRO, 56 gr FAT, and 5 gr Fiber. My lunch is the same every day, 2 individual packets of Hammer Perpetuem @ 5 oz, 516 cal, 108 gr CHO, 12 gr PRO, and 4 gr FAT. I mix one packet mid morning, and one mid afternoon, and just drip it in to help "burn" body/dietary fat. I never feel hungry until I stop in the late afternoon. A typical dinner is: 1)Stacy's Pita Chips @ 1 oz, 129 cal; 2) Annie's Cheddar Bunnies @ 1 oz, 147 cal; 3) Cheese Sticks(Whole Foods, again) @ 1.75 oz, 264 cal; 4) almonds @ 1 oz, 180 cal; 5) olive oil @ 1 oz, 250 cal; Nectar Nugget(Whole Foods) @ 1.1 oz,171 cal; Ultragen(a sports recovery drink made by Endurance Fuel Systems) @ 1 x 3.25 oz packet, 320 cal. I mix this and drink it immediately after stopping for the day to get it into my system within the 30 minute window for maximum absorption by the muscle cells to begin replenishing glycogen and repairing tissue. This day's worth of food weighs ~20 oz and provides 2754 cal, of which 1184(43%) are CHO, 292(11%) are PRO, and 1278(46%) are fat. I base this diet on the assumption that I am burning FAT/CHO/PRO at a ratio of ~65%/30%/5%, or 2730/1260/210. If I am getting 1278 cal of the 2730 from food, I have to get ~1450 from body fat; over 10 days that works out to 14500, which about equals the 4# of body fat that I mentioned earlier. If I am getting 1184 cal of CHO from my food, I have to get ~76 additional cal of CHO from elsewhere. It can come from either muscle glycogen, dietary protein or muscle protein. I think with this diet, it will come from that 292 cal of PRO I provide in my diet, either directly or via gluconeogenisis in the liver, leaving ~54 grams of PRO for body repair, hopefully enough for my 137# body weight. This is probably way more than you asked for, but I felt that nothing I said would make much sense if you didn't know the reasoning behing it, so take it for what it's worth and I hope it helps a little.
P.S. I take in ~60% of my fat at night when the digestive process does not compete with the leg muscles for energy substrate and O2. Also the digestive/repair processes produce heat, just like exercise; this is why exercise physiologists call them thermogenic processes. At night that heat can be put to use warming your sleep system instead of flaring off into the atmosphere as waste heat during the day. You can't stop entropy, but you can squeeze a little more use out of the inevitable loss of energy if you are careful.