A few thoughts:

1) As others have said, you really need longer sessions. An hour doesn't really hit your fat stores - few rides at longer distances would be better, even if you need to ride more slowly.

2) South beach is a decent base diet to consider, but remember that most diets are designed for sedentary people. If you start exercising a lot, you'll need to eat the "bad stuff" right before, during, and after the exercise. Probably not a big deal until you get to 90 minutes or so of reasonable intensity.