Walking is the best exercise. Do listen to other readers telling you to run and that walking is for the old or injured. Anyone who has ever used a modern treadmill can see the chart right on most units that lists a "fat burn" session as wildly different than running. The reason has to do with your heart rate. When walking, your heart rate increases but doesn't get too high. In this "zone" your body needs energy but the need isn't immediate or extreme. Therefore, it starts breaking down fat. When you start running, your heart rate goes up and your body needs an instant supply of energy. Fat doesn't covert to energy quickly enough for running, so you burn sugars and carbs instead of fat. Yes you may burn more calories running for an hour than walking, and that in turn may create a huge calorie deficit that helps you lose weight, but to specifically target fat - walk. There are dozens of sites on the internet that will tell you based on age/weight what your heart rate should be to burn fat so you can optimize your walking. That's why heart rate monitors exist and people serious about exercising use them.