Ideally, don't eat for about two hours before your walk. A gradual increase in both intensity and duration will ensure long term success. This will also greatly reduce the risk of injury. The best surfaces for walking are those which are designed for shock absorption. Running tracks, grass and dirt are ideal.
Concrete is harder on the joints and muscles and is therefore avoidable.
Try to include hills on your walks. This will increase your workload and the calories you burn. Keep your knees bent in order to reduce stress to the knee joint and prevent injury. Keep the other toes relaxed. This will strengthen and tone both your glutes and your low-back muscles.