1) Create a Big Sphere and stretch the terrain in this Sphere object.It will look like infinite.But not a real-time solution.
2) Action-based Terrain Loading:
Create a plane object.In this plane add a normal size terrain.Put your character on it and a SkyBox of course.While you see the end line of terrain add another terrain.The terrain will be added as you see the line just in time.and that will be a real Infinite Terrain.
For example:
We have 2-3 terrains and 3 cities.
When we travel from A city to B city if we see the end of terrain and the terrain that belongs to B city.To get to the C city when you see the end of the city of B add the terrain of which describes C City
Using this way,you will make a real-time solution to Infinite Terrain problem.
Cheers!