If you ride with a group, it's a great social acitivity as well. The group tend to motivate each other and you ride harder, faster, and burn more calories with a group vs. riding by yourself. I see bike riding as a hobby as I get more than just exercise from it.
Like all cardio exercise, you need to get your heart rate to your target zone and stay there for some time. Learn to pedle in higher cadence. Pick different routes and mix flat roads with hills. Hill is the best thing you have have - it motivates you and gives you sense of achievement. By improving your climbing, you will develop better riding technique, become stronger, and you will have to lose your weight.
As always, paying attention to your diet and calories intake is all important.
