Losing weight does have a lot to do with diet, but I've always found that exercise works best for me.
Walking is really good, but you have to keep changing things up, so your body doesn't get used to the same thing. During the winter, I do about 3-6 miles/day because I have a walking partner and we can get out, but during the summer, I'm more likely to hit the treadmill because hubby doesn't like me walking alone and it's really too hot to do a lot outside.