In this map, World of Attack on Titan , they show Paradis Island and the rest of the world. The walls are rings inside the Island, if there is a single layer of Titans, the width they could cover is the size of the walls. When they walk away of the island, they would separate and couldnt cover the whole Earth.
The only possibility is there are several layers of Titans in each wall, but I dont think that's what we saw or what the series meant, is it?
Were there several layers of Titans in each wall, or how is it possible they could cover the whole Earth?