People won't laugh but you may find it easier to wear lycra – its just more efficient on a raod bike.
You will need to get a lot of miles under your belt including hills, and assuming the ride is around 100 miles you should try and do at least one training ride of 80-90 miles.
Take appropriate clothing so you can adapt to any weather condition.
Also try out different energy gels, drinks and bars and only take what tastes nice and what doesn't upset your stomach – some people seem to have problems with some brands.
Don't start off too fast – as some people seem to do and then struggle on the first serious hill. Above all enjoy it!
@not enough time – at last year's Dragon ride there were a few non-road bikes including two guys on uni-cycles.