I purchased Minecraft PE - the full paid version, for something like $7. Not the lite or free version. How do I save worlds if I want to save them after I've already built them? The only option is Quit to Title. Is it only possible to create a saved, named, world when I first create it?
Quitting to the title automatically saves your world on every platform that Minecraft currently exists on except for console. Your world should be fine after you quit to the title.
In the lite version of Minecraft PE it's impossible to save worlds. That can be quite confusing when migrating to the full version.
In short just Quit to Title and your world should be saved.
Hope this helped.