Just try not to eat too much fat. Once you over eat the amount of fat your body can process that day, any more fat you eat is immediately stored as body fat. On the other hand, it is next to impossible to overeat the amount of carbs your body needs per day, since your body stores excess carbs as glycogen. Just look at fruitarians, they are lean as can be and all they do is eat unlimited sugar. Kenyans also get 70-80% of their calories from carbs and eat very little fat. From my experience as a runner who used to be fat as a kid, once I started making sure I didn't eat too much fat, just having a little bit a day(~30 grams), I slimmed all the way down before I even started running like 10 miles a week. Just my two cents though, I know many people will disagree with this form of diet. I just feel like a high carb, low fat diet is great for distance running since you need loads of energy to train (high carb) and a high power/weight ratio is everything in running so it helps to not have much fat to carry around.