Well I put 8.5 hrs to get round with a start time of 738 I will have to get a move on. 50 mile hilly ride on Saturday will give me a good guide on my time
I put the same time as you and also did a 50 mile hilly ride on Sunday - the Farnham Charity Bike Ride. I managed to do that, without busting a gut, in 3.39m and, given it was about 73°C (!) and quite an up and down route, was quite pleased.
Now, doing another 50 miles in a similar time is a whole different kettle of fish of course. However, one thing in our favour is the fact that the route is pretty level until Clandon with only a couple of draggy slopes to get up. What with that and the drafting effect of 19,999 other riders, I'm hopeful of feeling in far better shape at the foot of Newlands Corner than after 40 miles on Sunday.
How I feel after 65 miles is a whole different question of course!