"I'm looking at doing a 40 mile run/hike (in 12 hrs) this upcoming summer and I'm not liking the fact that my pack will be full of so much food (fuel).
Anyone have any insight?"
For a run of the duration you are considering, I would highly recommend you read the article in the link below before deciding on what food to use. It's focus is on ibuprofen, but the underlying topic is the effects of eating complex food that ties up the digestive tract while engaged in endurance activities. It boils down to a competition between the working muscles and the digestive tract for blood supply, and the digestive tract loses, with troubling results.
Based on this article and my own experience with distance running and backpacking, I would advise you to limit your nutrient intake to simpler carbs that require almost no digestion before clearing the digestive tract. In combination with body fat, which also requires no digestion, you should be able to supply your body with ample energy substrate with something like 1500 calories of something like Perpetuem or one of the numerous gels on the market, i.e. 15-20 oz of nutrient, plus some electrolytes. I use Morton's Lite salt, but there are many other ways of going about it. The amount of nutrient will depend on your weight, but this will put you in the ball park. Hopefully Greg gressel will post to give you even more detail.
Edited: Oops, Greg had already laid it out. In any case, the article is worth a read, for the potential side effects of not doing it his way, especially if you are considering using ibuprofen.