It can be done, but 21.1km is a big jump up from 12km, and you will need to be mentally prepared as well as physically.
You need to feel that once it starts to get hard that you can dig deep within yourself and finish. If you're finding 5km hard at the moment, that is a concern, but not insurmountable.
Do you run alone? Many people find running with a friend, or better still, a group of friends helps. Maybe find a local running group or club to help with motivation.
(Many a time I've organised to meet a mate or mates for a run, and you wake up, it's pouring rain and blowing a gale and you think "damn, xx is waiting for me, I'd better go" and when you get there you find they were thinking the same thing and then you figure, well we're here now, might as well get it over with and off you go).
If you let us know (roughly) what area you are in, someone here may know of an existing group in your area.
He's not the Messiah, he's a very naughty boy.