Welcome! Walking is a great exercise, but remember it is our nutrition that allows us to shrink the belly fat. It's often stated that 80% of our fat loss comes from following a healthy diet that allows for a slight daily deficit that will allow us to shrink the belly fat. So if you are not tracking your nutrition, that is definitely a starting point.
As for stretching...this is best reserved for after you are done with your walk when your muscles are warmed up.
Take care and keep on walking!