Mike,
Good luck with your fall marathon!
Which one are you doing?

I use GU energy gel, which is 100 calories of glycogen, basically. It really doesn't hurt to use this while training, at least I don't think so. It can help you run further and harder.
Also, I use Ultima Replenisher, which is like a sugar free gatorade. It has 35 calories per 20 ounces.

Judy