Hey Buckster -
Welcome to the forum! My ears were burning so I figured something must be going on here that I should check on.
I've completed 18 marathons, all of them under 2:30, and I won the Marine Corps Marathon in DC back in 2009.
Oh wait, that was just a dream I had recently.
In reality, I love to run but have only recently been able to - partly because of losing a lot of weight (the other part is a longer story). Doug and Umpa are probably much better qualified to advise you on the aspect of how exercise leads to an increased appetite.
I will say 2 things though -
1. Drink a lot of water. a LOT. Like 2-3 quarts a day (to me, that's a lot). Not only does it give your body the water that it really needs anyway, it also can take away a bit of the feelings of hunger.
2. Run early in the morning. I usually run around 6 am (here in Florida it's so hot this time of year that the early morning time is the most bearable). Also, you get your metabolism going early in the day and it runs for several hours, burning off the fat. Maybe someone else can come in with the science behind that. It works for me though.
But on to the more interesting stuff - what marathon are you going to run? Are you following any particular 20 week program? What time do you expect to finish in? I would love to do a marathon someday but right now I don't have the time to devote to the training. I do a few miles 3x a week just to maintain that level of fitness. A marathon requires quite a commitment, so kudos to you!
Scott