If you've done a couple of 33mile training runs, you'll be fine. The London to Brighton is not a difficult ride. You get so overawed by the occasion and all the riders, you almost forget the fact you're grinding away for 5 hours! There's just one sting in the tail! After about 50 miles you have to climb over the South Downs - it's not that long, just very steep. A lot of people walk it, but if you stick to a low gear you can heave yourself up in 10 mins or so, then it goes flat, THEN the mother of all downhills to the beach front!!!!
The one thing that you'll have to toughen up, is your [censored]! Do as many hours in the saddle as you can, and the big day will be a lot easier....... On the day, drink lots and lots of water, eat bananas, mix up some of that energy drink stuff, eat oaty flapjacky sort of things. The body can only really store 90mins of consumable energy, so you must keep drinking/eating, or your legs will go hollow - and that's no fun!
Hope it goes well....