15min is not enough. Ride for at least 25-30min. Technically your body was just getting started exercising. You need to exercise longer.
Eat a big breakfast, small lunch, and very small dinner. Eat small snacks in between. Make sure your meals and 3hrs apart though. And drink only water! That helps trust me, I've lost 50lbs without a dieting. I just ate small portions and only drank water.