Everybody is different. As you age your metabolism slows, as you gain weight your metabolism slows, then there's your diet and weather conditions that can affect metabolism. The best thing to do is make sure you are on a good diet (keeping a food diary can open up your eyes as to how many calories you are actually consuming) and trying different distances to see where you are at. If I don't do at least 40 miles three times a week I don't seem to lose any weight, and at 35 miles I basically just maintain. If I push it to the 50 mile range, then I seem to lose much more, but if it's windy or I do hills, then obviously I burn more calories, so try and find a good distance for you. Once you feel comfortable, then add no more than 10% increase in distance and do that for at least a couple of weeks to allow your body to adapt. You should find a point where you start losing nicely. They say 2 LBS per week is good, but sometimes I'm lucky to get 1/2 to 1 LBS no matter what. As long as the weight is going in the right direction, then it's working.