Not sure how the whole science of how the body works when we reach the 20+ mile stage but also interested to hear from anyone that can shed some light on it. I also suffer in the last 6 miles. For what its worth my blood glucose can drop between 4-6 units every 20 mins when running so it goes to show what is needed to be put back in over (for me) a 4 1/2 hour marathon.
'Fraid I cant offer really good advice as my dietitian freaks out with my pre race routine. I tend to start a bit higher than recommended (I know the problems with this - please don't judge). The key thing is good or bad (ideally good) do what suits you and by the time you have done the miles in training your body will tell you. I've tried drinks, gels, sugar sweets, jelly babies and combinations of all.
I have tried to be scientific by running on a treadmile and testing my B/S levels to see how much they drop every 20mins to give me an idea of how much I need to put back in. In the end it didn't change how I approached the race as I knew what got me round without having any problems. I don't keel over from lack of sugar or need to wee every 20 mins from being to high. Does me.
The one good piece of advice I can offer and it is a really obvious one, is find out what energy drink - if any - is supplied on the route and get used to drinking that. Easier for UK runs but I have done a couple in Europe and had to wait until the Expo to try it. There is nothing worse than turning up at a race to find either there is no drink or it tastes like anti-freeze especially if it is going to be part of your fuel top-up.
Others will have run more marathons and faster than me but if a fat, 40+ y-o, Type 1 diabetic can do it anyone can.