Here's a list of possible, non-perishable foods you could have prepared and ready to carry, together with their calorie-content per gram of weight:
Roasted peanuts - 6.00 cal/g
Crisps - 5.47 cal/g
Oats So Simple - 5.4 cal/g
Sugar - 5.0 cal/g
Powdered milk - 4.6 cal/g
Instant noodles - 4.4 cal/g
Peperami (long one) - 4.2 cal/g
Kit kat (2 finger) - 4.2 cal/g
Cereal bar - 4.2 cal/g
Plain porridge oats - 3.6 cal/g
Weetabix - 3.4 cal/g
Savoury rice - 3.2 cal/g
Cup-a-soup - 3.1 cal/g
White bread - 2.7 cal/g
Instant potato - 2.5 cal/g
Banana - 1 cal/g
Not an exhaustive list, I know, but hopefully illustrates the wide range of energy contents of different foods. So, it's much lighter to snack on peanuts and crisps rather than trying to take a sandwich for example! Also, noodles are more efficient than rice.
Specialist backpacking foods will tend to have higher cal/g ratings than the noodles, etc, mentioned above, but they are a lot more expensive.
If you're walking all day, on relatively level, firm ground, with a mid-weight pack, you're going to burn through 3000 calories plus. Of course, you don't need to replace all these, as many of us generally have some excess fat reserves
However, it is important to provide some fast- and slow-release carbs to stave off some of the problems mentioned above.Over a short 4-5 day bug-out episode, balanced nutrition isn't important. It's all about calories and fluid intake.
Hope this is useful.
