[Cliffradical]

Hard boiled eggs. High calorie-to-weight ratio. Shelf stable for a few days if the shell is unbroken. Roughly the same protein-to-fat ratio as your brain.
Not a replacement for protein bars, but protein bars and other "maybe food" will make you constipated and lethargic. Food bars are convenient, but they're a supplement, not a replacement. Try to pack as much real food as possible (a small orange acts like a cup of coffee after hour 12).
Caffeine makes you jittery, unfocused, dehydrated, then crash, and should be avoided. Then again, if you're a caff addict, going without will do the same thing.
Chew gum, grass, or a button to stay occupied and keep saliva flowing. Flex your calf muscles continually to keep blood flowing and stay awake while laying prone for long periods.
Get rid of foreign objects in your boots, readjust uncomfortable gear, and take a shit (!!! Use a designated pooper or bury your shit properly !!!) as often as possible. Rubbing, chafing, back pain, and holding dunks are incredibly quickly fatiguing. And distracting.

Don't talk, though. There is nothing in the world as good at dulling your senses and dividing your attention as a conversation.

Edit 2:
Please chew non-food objects responsibly.