I'm not out on Saturday Mark. I'm sure a 5-10 minute window is acceptable (you'd have left without me last week otherwise, had to go back for my pump
).
CSS to Brighton and back is about 90 miles I think, but you get a cafe stop at halfway to stuff in a few carbs on the beach
. I steady paced 19/20 and you should be fine, just sit in the wheels on the way down and don't do any work on the front, even if you feel okay. No one'll mind on your first CBC and it should make sure you have enough energy in the bank for the way back. Get a couple of energy bars to take with you (rather than mars bars) to nibble on.
The only real bugger of a hill is Ditchling Beacon but there's a re-group at the top before the final drop down to the sea front.