A few thoughts for first timers having read through the last few posts:
1. You will probably find your average speed is higher than your training rides. Adrenaline, closed roads and especially for me - better road surfaces all make a difference. The road surfaces in my corner of Northants vary from bad to terrible and really take it out of you on a long ride. Conditions generally much better on the RideLondon route.
2. Last year I started at 8.50, so pretty late. It was pretty relaxed, no racers, just people spinning along at a sensible pace, so quicker than me, some slower. In terms of number of riders, its not like any other Sportive. There won't be any time when you are on your own and out of site of other riders. I mostly rode on my own, but when I wanted company or needed a lift it was very easy to find someone to team up with to get through the hard miles.
3. The hubs and feed stations are proper busy, carry your own food if you can.
4. Leave something in the tank for the kick up through Wimbledon. It's not a big hill, but after 90 miles you certainly feel it. The good thing is that once you are t the top then it is down hill all the way. The last 10 miles is an amazing experience, enjoy it !