1. Insert > New Symbol and make it a graphic. Name it background.
2. Draw your background extra wide.
If you need to, google City Panorama or desert, beach, or whatever panorama to find a reference image.
Add a layer and drag your reference image into that layer, then Command-t to transform it.
3. When you are done, add a layer to your scene and drag your background symbol onto the scene.
4. Be sure to drag the background layer under the walk cycle layer
5. Create a motion tween to make your background move while the symbol walks.