Losing fat is more important then losing weight. But if you are new to cycling you will likely do both as you build lean muscle mass. I'm not an expert on interval training but I do know that getting your heart rate high is more important then long duration. You will lose more weight climbing hills for 30 minutes a day then a 2 1/2 hour ride at moderate speed. Same goes for running. The most important thing I have learned from experience is pacing yourself so as not to let yourself get burned out fast and you lose the ambition to continue. Just get a steady routine going.