FAT!

http://www.meandmydiabetes.com/2012/08/11/western-states-100-low-carber-wins-ultramarathon-steve-phinney-and-jeff-volek-study/

(Actually I'm sure that he replenishes his glycogen as he runs, but he bases his diet around fat).

You should listen to Tim Noakes (author of the brilliant "Lore of Running") on how he has repented his former advice that carbohydrates are necessary for marathons and ultras:

http://livinlavidalowcarb.com/blog/the-llvlc-show-episode-588-south-african-running-legend-tim-noakes-embraces-high-fat-low-carb-living/14565

Also check out Volek and Phinney's "The Art and Science of Low Carb Performance".

It's hardly mainstream, but I'd be willing to bet that there are more endurance runners low-carb/high-fatting than are prepared to admit it, for reasons of either humiliation or tactical advantage.

I train most of my runs up to 60 minutes fasted. I think it helps train your body to burn fat efficiently.

For my long runs (greater than an hour) I eat a banana before the run, and bring a coconut water drink (especially if it's hot). I drink the coconut water right after the run (its better than gatorade) or during the run for really hot day or especially long runs.

I do GORUCK Challenges. They last approximately 12 hours. Pre-challenge I over-hydrate with water and coconut water. I eat a big, fatty meal 4-6 hours before the challenge. I down a can of coconut milk an hour or two before the start, a banana just before, and use home-made fruit and nut bars as snacks during the challenge. I've never needed a "GU", cliff-bar, or 5-hour energy, which many of my fellow challengers are consuming like candy.

I experimented with making my own gels a while back and used a recipe from Brendan Brazier's site. He's a (ssshhhhh) vegan triathlete, but his gel recipe was, if I remember rightly, dates, coconut oil, lemon/orange zest, raw cocoa powder and poss some agave, which you could replace with honey, all blended together. So, v. paleo. Should be able to find recipe via google. They were effing delicious (chocolate orange!) and my only problem was finding a suitable recepticle to carry them in. On my long bike rides I'd use them and nakd bars, which are the perfect size and just dried fruits and nuts.

If that's too much faff, you could just take sachets of honey? Or train yourself to go without fuel - I have a few mates who've managed to do that training for ironman.

I would love to comment about how bananas are comparable to gels, because I've read a lot of studies that show evidence that bananas are in fact just as effective as sports drinks. However, I use Gu Roctane, I won't lie. I consume 4 gels during a 3:20 marathon. I like them, they work, I don't experience any GI/IBS from them and I'm the idiot who figures that running marathon isn't paleo anyways.

I've found dates to be a very portable, gu-like, whole food alternative. I swear they taste just like a gel if you keep them in your mouth long enough.