Invite. It's not a paid account feature or anything.
...Uh-oh. I just tried adjusting that setting to add a few more Previous pages, and that made the Previous 20 link disappeared from my page. Changing it back to the old setting didn't fix it, so I guess it's a bug, not intentional. Crap. Only clicking "reset to defaults" brings back the link (and it only goes one page back).