From what I experienced last year since I started running...during the first 2 months my weight dove from 180lbs to 160lbs...i also did some weight training...during the start of the 3rd month I increased my mileage and became more strict on my diet but my weight started to plateau.I consulted my gym instructor and he told me that it's normal because my body is already used to the diet and exercise. He said that this is where the real weight loss starts. This is where you start burning some real fat and build some muscle. Even if I do 500 sit ups everyday I wouldn't still be able to have those six packs if I don't change my diet which is to avoid the 3 white food (rice, salt, & sugar) which are the culprits to weight gain.