Incredibly well organised.
As for group I think you are selling yourself short. I used to train with friends at 16.5-17 mph and was OK in Group 2 at 19mph.
I changed my weekend warrior rides and go with my local club. We come in at 18.5-19.5. Mph. When doing Ride London 100 I came in around 21 mph average speed.
So my estimate is to add 1.5-2mph for closed roads rides.
Plus, in terms of distance, lunch breaks are about 45mins long so you normally ride 60-70 miles in the morning and 30-40 miles in the afternoon. It's not like doing a century sportive.
So regarding training build up the number of times you cycle longer distances on consecutive days.