1. Get comfy! Saddle and bike comfort is very important as you will be pedalling for at least four hours to complete the ride. You need a good pair of shorts with a padded support and also need to make sure the balance of your weight on the bike between the handlebars, saddle and pedals is right. A good local bike shop will help you with all these things and ask them about buying "chamois cream" for your shorts, which is very important on long rides.
2. Pace yourself. This is important with your preparation for the event and the event itself. Gradually build up your miles on the bike during your preparation phase. During the ride itself remember this is not a race. You will finish strongly if you go a bit easier than you think you need to at the start.
3. Fuel correctly. Eating and drinking are both of the upmost important during a ride of this length and you may need to practice doing this on the bike. If not whilst moving, then remember to take plenty of short breaks to get the food and fluid inside you. Drinking before you a thirsty is very important and eating something small every half hour will stop you getting hunger pangs or worse still running out of energy altogether.
4. Always wear a helmet and do all your practice and preparation rides using the helmet too. It might well save your life one day.
5. Enjoy it! This is the most important part for whenever you get out on a bike. Enjoy the feeling of freedom, the fresh air and the beautiful countryside.