Large terrains for flight games

Hi!
What would be the best approach to creating large terrains for a flight game similar to HAWX, Ace Combat, etc? The majority of tutorials that I've found consider terrains that are large from the perspective of a walking player, not airplane.
What assets/software would be best to tackle this problem, both in terms of terrain geometry and textures, and why? I'd prefer not to use real locations.

Example of what I want to achieve:

 

Unity supports adding any number of terrain objects to your scene. The World Streamer asset is something you could consider for streaming in terrain as you move around the map. Also, you may want to use a floating origin system to avoid float accuracy jitter when far from the scene origin.