Dynamically expanding existing Level by adding "Sub-Levels"

Hi there,

I have an arcade style game, that is if we reduce it to a lower level nothing else than a race from point A to point B.

So everything that is behind the player won't be needed anymore, and everything that is to far away shouldn't be loaded until it is needed.

I want to have the rough structure of the level loaded once completely (curves, straight lines, loopings => "placeholders"), and if the player comes in range of a certain part of the level it gets loaded, and the part behind him gets unloaded.

I tried to do this by using scenes, but I do not like to mark that many objects as "persistant", because actually I have one scene and I am un/loading others into it. There is a breach in my personal sense of logic, because when I load a scene, this scene is the "main" scene I am currently in, but logically this is according to my concept wrong, because what I loaded is just a part of "the level" and not the "level" itself.

Any ideas?