The weekend before the Event, try to do a reasonably long ride as a test. Try to do one long ride each week. Maybe 3 hours then 4 hours and then 5 hours or something like that. This is a pretty aggressive progression but it sounds like you are athletic and this might be fine. If you were a couch potato, it would be impossible.
To have a chance at 200km, you should be able to do about 130 km (65%) on that preceding weekend. A more reasonable target might be 100km and then do an Event distance more like 140-150km. That would be a "stretch" goal in my mind.