I don't know what sort of speed you would be riding at, whether it is a competitive ride or not but all I can suggest is that you get as many miles as you can before the big day (obviously). 2-3 longer rides a week, 2-3 shorter rides at a higher intensity and 1 shorter ride in the biggest gear you can push, to help build strength (saves time and money wasted on lifting weights at the gym). Obviuosly those distances of shorter and longer depend entirely on your current level of fitness, and how well you recover....