By all means go out and ride a fairly long ride this weekend and the week before your long charity ride go ahead and do your 20 milers on the weekdays.
I would recommend you don't burn yourself out just before hte ride - so go easy hte week before on those 20 milers especially towards the end of hte week and take of the day before or maybe even both thursday and friday so you are fresh for your ride on saturday, if it is on sat?
Have fun - go a little slower - you will be fine When you are training say for a 100 miler, you don't ever have to ride the actual 100 miles before same with your shorter ride. You will be fine