First of all - congratulations! Not only have you set a goal, but you will be helping out charity too! I still consider myself a new cyclist because I've only been doing it since April 2004, but it is the best exercise.
I have a few tips for you:
1) Try to set smaller goals between now and then
2) Be consistent with your training - eventually doing a long ride (30, 40, 60, 100 miles depending on where you are in your training) at least once a month and pick out nice places to go and even working up from there.
3) Try to hook up with a cycling club or ride with a partner (mine is my husband). It's easier to stay motivated when you have someone to do it with.
4) Remember that it's not a race - you can go as slow or as fast as you want
5) Properly fuel your body - for long distances carbohydrates are your friends
6) invest in a good pair of padded shorts or a gel seat...it's a long time to be in the saddle
My husband and I started out by doing small 5 mile rides (those were killer to us at the start) every other day and venturing out to 10 miles on the weekend after we'd become acclimated (didn't realize we had those muscles in our legs!). We now do 10 miles/day every other weekday and then 20-40 miles each weekend day. Once a month we pick a place to go (with guidebooks we've purchased) and do between a 60-100 miler. I never thought it would have been possible at the begininning!
Good luck and have fun!