I have a book by Richard Hawley who as been active on SimHQ (SimHQ was even mentioned in the "Thank you" section but I'm not sure of his current status since I haven't been as active as I used to) called, "Grome Terrain Modeling with Ogre3D, UDK and Unity3D" with a tag of "Create massive terrains and export them to the most popular game engines. It's from 2013 so I'm not sure how current things are but Google has a copy you can look through here:
I haven't had a reason to deal with large terrain in Unity myself so never got around to really exploring it.
Searching on "unity large terrain" gives me hits of other people trying to deal with this but I'm guessing you've probably looked through some of these.
Of course the original F-19 was so limited it wasn't going to get anywhere near the Uncanny Valley in regards to realism. For now I think you have the right idea of focusing on fun instead of realistic scale.
Elf
I wonder what happened to 'Combat Helo' that Richard Hawley was working on. It was using the Leadwerks engine and was looking very promising, although I suspect that it would be subject to similar large world problems with that engine.
