A good friend of mine who happens to be a kick ass triathelete, ultra-maranother, and marathoner (slacker), turned me on to Generation UCan  for fuel during my training runs and marathons. This friend is so fast and knowledgeable that whenever *she* tells you to do something, you do it.
So what is it? UCan is a slow release carbohydrate, super starch that keeps blood sugars stable and delivers steady energy to your brain and body (vs a burn and crash). It prevents the spike and crash of sugar-based fuels and is gentle on the stomach. Two things that are very important for fuel in a marathon.
It was first created for diabetics and they then realized that it would work for endurance athletes. I used it in the Paris Marathon  this year and was happy with the outcome (no stomach issues, steady full feeling till halfway through the run), and so now I plan to use it for Chicago.
You do need to mix it into water and blend it well so it tastes good. The best strategy to prevent it from being clumpy (and ok tasting) is to buy the ucan mixer  (worth the 10 dollars), which you mix in the night before, and refrigerate overnight. The Ucan with protein is best for a recovery drink which you can blend into a smoothie.
To read reviews and more of the science behind UCan, go here  and here  and here . For recipe ideas (YUP), go here . In any case, I am very happy with my fuel situation right now on my long runs and upcoming marathon — Thank you, Jan, for introducing me to this drink — and so happy it is my go-to these days!