The default config should make the horde starting in a random location, you just have to select the right preset in TerrainLimits.xml to be sure it will not spawn in the ocean.

Note that once spawned the horde will not choose another location when the server restart (it chose a new location only when it respawn after have been defeated)
If you want a random location at each restart you have to set the "Persistant_Position_When_Server_Restart" to 1 in MainOptions.xml