In reply to Dave:
As others have said, gels are harsh on the stomach. You get far too much highly available sugar in at once, so your stomach has to draw water in from elsewhere in order to process it. This is OK for a while, but if you keep asking your body to divert precious water resources while it's under sustained pressure to make that it available to the rest of your system, eventually the whole process will fall over.
Solid foods don't have the same problem as the slower digestion means there is much less water required at any given time.
Most of the commercial energy drink powders are not that great either, as they come with too many short chain sugars - which means if you mix a drink with enough energy density, you'll still have to keep drawing water back in. Also they tend to be flavoured to taste nice for someone doing an hour or two of exercise, and after three or four hours of working hard that starts to become nauseatingly strong.
If you do want to avoid carrying too much real food, working out your hourly water, electrolyte and fuel requirements, and creating a custom powder formula, for me was the gold standard. I've only gone up to about 8 hours that way but it was the only strategy I found for those sorts of distances that guaranteed me no GI distress whatsoever. There used to be a company called infinit who did the customisation for you once you'd worked out your requirements, but they're no longer operating in the UK, so a bit of geekery would be required.
