Some roll a golf ball under their foot, or draw the alphabet with your toes a few times a day to move the foot muscles fully, some buy some sort of weird sock you can wear at night that helps. (If it's that).
You are probably better off just paying to go to a sports physio and hopefully getting a good diagnosis and treatment plan straight off.
It's only a week before, so no point doing 20, just settle down get a decent 6 or 7 mile run in relax and feel great. Going to far and being injured or tired just isn't worth it, you've done the training now it's just a case of getting to the start line fresh and in one piece.
sounds good Matt. All systems go watch the marsthon Sunday to get the excitement levels even higher and then all systems go. Just remember to chill and relax now, you cannot improve from now on, so don't do what I do and get injured in the taper, just take it seriously easy and enjoy it.