I've lost 65 lbs. doing 15 mile rides on a flat creekside trail 5 days a week. At first, I didn't lose anything and was stumped. My wife pointed out that I needed to cut down on the food too. When I did, the pounds began coming off. The other thing that helped was 'spinning'. Ride in an easier gear but pedal more quickly. This gets the heart rate up big time, and you enter the calorie burning stage.
It takes time, but is enjoyable and you will be in good shape in no time. Good luck. bk