Try making your own isotonic energy drink with 50/50 fruit juice/water and a pinch of salt. Much cheaper than commercial drinks and works just as well, I feel. 1 pint per hour delivers about 30g of carbohydrate, which is what you need if you're running more than 2 hours. Start drinking 1/2 hour into your run, at the latest, and then drink at least 1 pint per hour. You can condition yourself to do this during training - don't just try and do this on marathon day. Not only does it keep you hydrated/energised during your run, it also helps you to recover faster too. If you're trying to lose weight, not drinking during your run is an inadvisable way of doing it. Weigh yourself before you start running and then again immediately afterwards (assuming you don't drink anything during). The difference in weight is fluid loss which you will regain as soon as you take a drink. What you need to lose is fat, which can best be achieved by a calorie controlled diet with the correct ratio of carbohydrate/protein/fat. I know proper hydration/carbo loading works. My marathon PB of 3hrs 16mins stood for 10 years when I drank only water. After advice from an old cycling/triathlon friend and a switch to the 50/50 juice/water, on about the same training I ran a 2 58. Suck it and see. A great book on sports nutrition and weight loss loss is Anita Bean's Sports Nutrition. I trained hard for many years, but finally discovered that it's training smart that gets results. Good luck with losing weight.