First of all, congratulations on deciding to do a marathon!
I think changes to diet and exercise should both be made gradually to minimize shock to ones body. One of the first things I did was read everything under the Training Corner section of this website and then get a good book on running marathons. There are many out there for different types of runners. Id suggest going to a bookstore or library so you can look through it and find the one that suits you. A good one will include a section on nutrition as well as how to build up your mileage.
You'll see that a high carb diet is generally recommended for endurance athletes, i.e., 60-70% calories from carbs, 15-20% calories from protein (or 0.5-0.75 gm per pound body wt), 20-25% from fat ranges. As mileage gradually increases, calories need to increase as well while trying to maintain the C-P-F ratios (do the opposite during taper period). If you are interested in losing weight, however, then youll need to try to achieve an overall calorie deficit instead of a balance, which can be tricky when building up mileage to do a marathon. In this case, Id suggest getting a sports nutrition book (such as Nancy Clarks Sports Nutrition Guidebook, Ellen Colemans Eating for Endurance, Suzanne Girard Eberle Endurance Sports Nutrition) to get a more indepth understanding of nutritions role in sports performance. I also take a multivitamin as well as an iron and calcium supplement since I do not eat much meat or dairy products and Im over 40.
As for hydration, the article mentioned in my 6/30/03 post below in the 2003 issue of Runners World has got some relatively new findings that may not be in some books. Feel free to ask more questions if you need further help. I hope others will chime in if they have other ideas.
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