Well, for comparison, I did a 20 miler at the weekend too, and drank 300ml of water and 300ml of a 4:1 carb/protein mix I was trying out. Ran out at 18 miles and would have liked some more water but it was a REALLY hot day. Normally two 300ml bottles will get me round 20 miles with some to spare. Through the winter I'll often get home and still have half left.
I also didn't take any food till the halfway point, which was a jam sandwich - at about 2 hours like Tim R says. Then took a caffeine gel at about 3 hours for a bit of a boost, but that was all. I do use gels more than food in actual races, cause when I'm running faster the gels go down easier, but I still probably wouldn't start using them before at least 10 miles in a marathon.
I think Tim R's advice is good. Are you actually slowing down, feeling dehydrated and thirsty, feeling hungry and running out of energy? Or are you just worried because you think you should be eating/drinking but your body doesn't want to cooperate?