How to extrude terrain to infinity. Do you want to have your terrain as real terrain without any water around it?
If you want infinite terrain, then you must extend the terrain elevations into the edge of the map, then it will be extruded to infinty from this point. If you have 5m elevation in there, then there will be 5m elevation terrain to infinity.
It would help if the terrain would be somewhat even couple of cells before the edge, so the transition to infinite terrain would look smooth.