I was able to lose 40 pounds while pedaling my big behind 5 years ago. It is just like any other exercise in that you still need to do it for periods of time. Unfortunately a single exercise period can take hours. Most people do not raise their heart rates into a cardio range like they do on other pieces of exercise equipment. None the less I still love riding my bike. I started by riding my mountain bike on a path 4-6 days a week, building up to 19 miles each time out. I got into a nice routine and my wife started joining me too. We now each have mt and road bikes, riding fairly regularly. We did get to a point where it seemed our cardio was stagnant, so we supplemented it with running, then swimming. Riding my bike is now supplemental to my other exercises I do, but pedaling is still my favorite.
Like all new exercises and being that you have not ridden in 10 years, take baby steps by going maybe a mile or 2, and add a little each time as you feel comfortable. You will develop a routine and your favorite routes. If this is going to be your primary exercise, I recommend you do it as often someone else may go on a treadmill (or other piece of exercise equipment).
You will want to make sure your bike is in good working condition (a local bike shop can help with this). I also recommend wearing your helmet all the time. I know they make most of us look goofy, but we will look a lot worse if there was an accident with no helmet.
Todd

Turn the motor off!!