Now it is very possible to run a marathon without eating. The only time I have run 26 miles I only ate one gel and drank eight ounces of water and I finished the run quite strong. However, in general trying to complete a multi-day backpacking trip in one day of running involves more up and down than your typical road run. It might also involve higher altitudes, which raise your metabolic rate, so you need to eat more. It could also involve in-climate weather and eating helps keep you warm.
I had energy bars for the first time when I started backpacking in 2001. I started using energy gels in January 2008 for my long runs. One of the slowest things that you can plan to do when "running" a trail is plan to cook or sit and eat. If you can't eat it while you are running or at least walking, it is too complex. The keys to food choice are taste, digestibility, and simplicity.
You have to like, or at least tolerate well, what you are eating. Otherwise there is a very good chance that you will get to a point where you just do not want to eat what you have. Then you are carrying dead weight and you are not getting the calories you need. It has to be digestible. Energy gels are a prefect example of easy to digest. Blocks of cheese and meat on the other hand take a little more effort. Additionally, it has to be easy. If you plan to eat a sandwich, make it before you go instead of carting the ingredients separately. Cold, bloated, shaking, and sweating hands will make it difficult to make a sandwich at mile 35. I have eaten a bag of potato chips while running and it was hard to breathe and chew without choking. Cliff Shots have a Litter Leash that keeps the top of the energy gel attached to the packet so that there is only one pice of trash. One piece trash makes things simple.
It is also important to eat at consistent intervals. This could be as simple as eat an energy gel every 45 minutes. It could be eat when you feel like. It is simply important to keep eating. Try not to let yourself go a few hours without eating or your body may choose to shut down for some undesirable length of time.
Finally, keep an eye on nutrition. You need mostly carbohydrates to run. You will also need sodium and potassium to avoid cramps or hyponatremia. While it is important to eat what you like, so that you eat something, salty carbs are at the top of the list for beneficial ultra running foods.
Some of my recommendations to try are the chewables, like Cliff Shot Bloks, Jelly Belly Sport Beans, GU Chomps. Try energy gels if you have not. Some of the flavors are not very good but other flavors taste better than jelly. Chocolate almond butter might be my new favorite. I have eaten the protein bars while on runs and I find them too dense to consume while I run. They are fine when I am sitting and great after I am done but during a run the protein is just too heavy. Ultimately, you have to try the different brands and flavors to find out what works best for you. Enjoy shopping!