If your young enough, pretend you want to join The Marines and go on the pre-selection course !
I did the endurance run as a Naval cadet over 30 years ago and it hasn't changed much as far as I can see - except I don't remember an instructor manning a sluice valve (minute 10) ready to empty the entire submerged tunnel obstacle when I did it - that's Health & Safety for you!
From all I've seen, Tough Guy (the original, despite Tough Mudders claims) is bay far the closest you will get otherwise.
The majority (ovbservation from working on aid stations) do not mix hydration and nutrition; if they put anything in their water, it is a zero-calorie electrolyte tablet (Nunn, Zero etc.) or Elete (comes in a dropper bottle and is easily the most economical particularly in bulk); Of these, Elete gives you the least hassle cleaning bladders etc.
There are several reasons for not mixing hydration and nutrition - harder to manage and the fact that you have nothing to drink if your body starts rejecting carbs being the main ones; also, you can suddenly decide that the taste of your only hydration source is suddenly repellant, when it was fine 10 minutes ago (your mind and body just decide things like that.
The other option is pure water; Noakes suggests that you don't actually need suplementary electrolytes while running, but I've not met anyone who's had the balls to put that to the test! More conventional would be to take electrolyte capsules such as S-Caps.
I posted this in another thread a while back; I'd regard this as a suplement to commercial gels (not as energy dense, but more constant) - I still use cliff gels, but less of them + I don't use gels on most training runs (the cost issue!):
"... and Chia; I use them as well as Gel, particularly earlier on in a race - they are claimed to have a slow-release affect on carbs (they are a reasonable source of carbs themselves) and as I naturally tend towards being hypoglycemic, anything that smooths out boom-or-bust insulin spikes is welcome!
If you are interested in trying it (quantities will slightly overfill a typical gel flask) soak 4 teaspoons of seeds in 4 fl oz of water (best in the fridge, overnight) and add one teaspoon of rice syrup and two of agave syrup, with a splash of cordial (Bottle Green Ginger & Lemongrass is great, but Ribena works OK) - I also add Elete for electrolytes, but that's personal preference. The texture is a little like frog spawn, but it really is quite palatable!"