Imagine you haven't played football for a while and someone asks you to play a 90 minute game tomorrow. Now would you immediately go and play for 70 minutes to better prepare you?
The psychological edge you think you will gain will be lost by the well founded concern over how fresh your legs will be so unless you are an endurance monster I would advise a shorter run (Hal Higdons programme advises a 12 miler today but I have seen 13 in some other plans.)
All coaches will say that you cant improve anything the last week but you can make matters a whole lot worse. Whatever you decide best of luck for your event
As Millsy says you could probably do it given your background of running and lasting a twenty miler but you would have to be discpilined in your pace. Why not just enjoy the run without having any pressure timewise. You could even take in the sights that would normally just be a blur to you!