I did the South Downs Randonee 2 years ago from Winchester to Eastbourne - bloody great fun.
Advice I would offer:
- strength and conditioning training helped me immensely - kettlebells, circuit training etc Get your glutes and back as strong as you can.
- train in all weather conditions, I was fortunate to have a dry day and a westerly behind me on the day itself but it could have been a whole lot worse.
- flapjacks, jelly beans and water
Good luck, I'll stick some money in the pot before the ride.