I agree that much of weight loss is diet, not exercise, but that being said:
Cardio ups your resting metabolism for a few hours after you do it. It can burn a couple of extra calories a day. Not alot, but any will help. Same with doing the actual exercise. You may be suprised by how little you burn if you look on a website like exrx.net, but every little bit counts.
Hands down, the best way to get the BEST results for running is interval training. On a 1-10 scale, with 10 being the hardest, do about 2 or 3 minutes working at like a 5 or 6, and 2 or 3 minutes at an 8 or 9. Try this for 30 minutes, 5 times a day.
THey did research on running and it turns out that the positive training effect really minimizes if you do any more than 5 times a day, and the chance of injury goes up dramatically. But to be healthy, you need to do at least 3 times a day. I'd say your best bet is 4-5 days/week for AT LEAST 30 minutes a day!
Also, as far as that "fat burning zone" thing they talk about on treadmills. That's a myth. Work as hard and as long as you can. That's the best result.