Wanting to do charity bike ride but very nervous

Hello everyone
My name's Jaq and i'm from Devon in England. I have been cycling daily for about 18 months but i'm a bit of an ambler. I'd really like to improve my cycling and fitness to be able to do a charity bike ride for The British Heart Foundation in 2012 from Vietnam to Cambodia. However, I am overweight so i seriously need to improve my fitness and I just wouldn't know where to begin with training. I have asked a few friends if they would be interested in doing the challenge with me but so far there have been no takers :(
Can anyone advise me on how to get started? Should i start with a specific distance and slowly increase this? If so, by how much and how soon? How many days should i be cycling?
So many questions - sorry :o
Can anyone help?