Andy Miner's round Britian charity ride
i agree with the previous comment that you need to be sure you are able to tackle the daily distances on a continuous basis and cope with the weather.
For our rides through South Africa, we trained for about 18 months, but of course we were in our late 60's at the time.
If anyone is interested, I have written a book about our experiences. It is called "I've Grown customed to His Butt".