Back when I was running marathons - the drink I liked best was Heed. It isn’t nearly as sweet as some- I often mix a little weak.
I would drink every time there was a water ‘stand’ and of course drink lots of fluids several days leading up to the event. Deninately made a huge difference. (Of course if it was cool and raining, I did much better also). Generally used GU about as often during the event and seemed to help keep me going.
Mark