Thanks for summarising to BenB.
So that maths lines up with around 800kJ for an hour running on my Garmin and on a calculator on the fellrnr site. That calculator also shows how much less energy is needed for ‘normal’ weight people, blimey.
I think that gives me confidence Im fuelling well enough with two 26g gels an hour.
The rest is training and health. And water. I lose a kilo in the first hour of any run.
Thanks for helping me think this through.
FWIW I’ve run two half marathons in 1h50, but fell apart in two marathons…5hrs which I didn’t fuel. If I do it again I’ll be “one of those guys” with a hydration jacket covered in gels 
I weigh 86kg where a normal weight would be 65kg. I’m not weak, I’m not unfit, but my body fat is only going to increase if the last five years are anything to go by.
I trained up to do a 30km run last autumn but bailed on it didn’t run farther than 21km