For your lunch, just prepare something more basic or easier than your dinner. You can carry some pre cooked hamburger with taco seasoning; you can also bring some cheese with mixed vegetables for a quesadilla, or some tortillas.
Lunch should be kept simple and cook something that is easy to prepare.
I am sure you will not have the energy to make a big meal. Probably, think along the lines of sandwiches and finger food. Around lunchtime, you will be most likely feeling thirsty for something else other than water. You can carry clear water containers with teabags to make sun-made iced tea. Bring lemon together with some sugar cubes and you will enjoy. You can also bring lemonade.