If you get dizzy, first you should check this out with your doctor, and secondly, you should probably put off the commuting for now and just ride your bike recreationally to gradually become more fit. It probably won't take much riding to get to the point where you can ride 12 miles easily if you're otherwise healthy. There is no supplement that will do this for you, and you will just be wasting your money. There's no such thing as bicycling nutrition unless you start talking some pretty serious racing, and maybe touring. But even serious tourists really only need to eat enough, that's all. A heart-healthy diet is probably a good idea for people our age (or any age for that matter).