"Render Distance" Job-ee; How would i go about adding a "render distance"?

Im currently working on this game and i'd like to create a sort of render distance, basically only the parts of the map that are within a set distance of the player are rendered, meanwhile, everything that is not rendered will act the same as if it is being rendered (meaning if a player is out of his/her rendering distance from say... an enemy, that enemy will continue to move/act how it would normally act, etc)
I only want to do this because the map is going to cover about 3,000 square miles of land all at once (at least, whenever the player is out side it will be like this), so i'd like it to not lag the hell out of their system.
If this is not do-able, then is it possible for me to have say ... 300 (10 square miles per scene) scenes for, each for a section of the map, and every scene has a boundary where if they hit the boundary, it will put them in the scene adjacent to the side that they were at?

Thanks a bunch!
~Tristen

 

http://docs.unity3d.com/Documentation/ScriptReference/Camera.html

Far clip plane


ohh just stumbled on this while I was in there... neatness
http://docs.unity3d.com/Documentation/ScriptReference/Camera-layerCullDistances.html