Most of my runs I don't really need to worry about fueling, just water. This summer was exceptionally hot and dry, so I brought water on almost every run. And that's OK with me; my pack gives extra room for carrying my phone and camera, keys, etc..
However, with all the longer runs I have been doing recently, fueling has been essential.
It's just not going to be pretty if you are running all day on the trails and don't have anything to keep you going.
I believe sugar is essential (carb), though some might prefer to add fats or protein to fuel with. The bottom line is that you need SOMETHING out there.. Figuring what the right combination of "something" is- that part can be tricky.
My personal observation is that in a race where you expect to run the whole thing without using the restroom (big potty, here!), it's best done on strictly non-fiber carbs (gels, sports drinks, beans, chews, etc..). This is best for races under 5-6 hours.
Fiber and protein (i.e. bars, meat sticks, trail butter, real food), these things are known to cause "big potty". They also redirect your blood from elsewhere in the body to your stomach for digestion, and will lead to a stop or two in many people's races. They also, however, provide energy to keep a runner going longer and that is why ultra runners must have them if they are running for hours on end. (Only going on research here, since my longest run ever was not quite 6 hours).
After running out of some prepackaged bars, I decided that my 5 hour runs would need some alternative fuel, and I went to the kitchen to see what I could whip up.
This is what I came up with:
