Facebook has a big shiny button that allows you to do a big change in one go!

Limit the Audience for Old Posts:

Very simple this one, if selected all your Public and Friends of Friends become visible to just your Friends.
From almost any page, select the drop down arrow at the top right and select Privacy Settings.

One of the options is clearly is verbosely named “Limit the Audience for Old Posts“.

Select this the you’re prompted, just to make sure.

It is very important that you are sure as this cannot be undone easily. If you’ve many posts over many years it’s essentially impossible.
Also note that it is still possible for people to view some of your older posts, if they have been tagged in a Photo or Check In for example.
If you are sure, then click the “Limit Old Posts” button.
Once again you’re prompted to confirm the action.

Now all your posts are visible only to your Friends.