The key to fat loss is boosting your metabolism. Walking increases your metabolic rate and heart rate, which in turn increases oxygen and burns calories. If you walk to lose belly fat, you need to walk at a steady speed fast enough so your heart beats faster and causes you to breathe harder and deeper.
The only way to lose body fat, especially belly fat, is by burning more calories than you consume. A well-balanced diet, along with a regular walking routine, will help you lose belly fat within a few weeks.