The simplest hack way to set up the terrains for Unity is to use/duplicate the example TerrainGenerator.hda from the Houdini Engine integration "Assets\Plugins\HoudiniEngineUnity\HDAs". For a very quick explanation of the setup and how to make larger worlds is to follow the steps here https://www.sidefx.com/docs/unity/_terrain.html . This following citation does not use PDG, but it will get you going quickly.