Okay, I've got a few minutes, let's see if I can explain this in a way you can get your head around. I've implemented this myself on a far larger scale so I know it well.
As it stands going further than 10km from the centre of the map causes issues with the character shaking, the further you get from centre the worse it becomes. This is mostly caused by unity's strange insistence on using "float" instead of "double" for the main playing area. This would still happen if they used doubles but you'd have a far larger distance before the shaking occurs.
The way to solve this is that when the character gets too far from the centre of the map you then redraw the map with the characters current location at the centre.
For example, you travel West for 10km, before the shaking becomes a real issue. At that point the map is redrawn but offset 10km West, you're still 10km from the original centre of the map but the players character is in the centre of the playing area where the shaking doesn't occur.
You can hide the redrawing of the map from the player and, with a bit of programming skill, the transition should be largely unnoticable as the only things that really need to be moved are the player and any Z's that may be around.
I managed to generate a 1,000,000 km wide map using this technique before the libnoise library started to break down, it simply couldn't calculate the map with enough accuracy. 1 trillion square kilometres of terrain... damn... I _still_ like hearing the sound of that