Simon Doughty's Long Distance Cyclists Handbook is a worthwhile read:-
http://www.amazon.co.uk/Long-Distance-Cyclists-Handbook/dp/0713668326
ISBN: 0713668326
ISBN-13: 978-0713668322
For someone considering it in 2013 I'd suggest a minimum of:-
a) Start doing Audaxes now (if you're not already doing them). You can dip your toe in with a 100 if you like, everyone starts somewhere. Just start.
b) Ideally you'd work up to 200km rides within a few months and keep that going over the winter, there are some lovely winter 200s in the calendar.
Don't worry too much if you don't, but you should be happily doing a 200km ride by Feb/March 2012.
Also don't worry if you finish close to the time limit on a winter ride. I'm near the back of the field usually and will took ~13 hours for a winter 200 in the run up to LEL'09. Even at my fastest in the summer I'm still taking at least 10 hours for a 200. For the longer rides I've rarely finished with more than an hour to spare, and minimal sleep.
c) Aim for an SR (200, 300, 400, 600km) in 2012.
An SR in 2012 would be perfect, but not absolutely required, preparation. There are plenty of people that didn't do any long distance riding until 6 months before LEL 2009. However, the more prepared you are the less likely you are to fail.
What you almost might find is that night riding just isn't for you. If you're not fast enough to minimise it then you may find that rides longer than 300km just aren't for you (I know several people who've found this), but give it a few tries before deciding this.
The SR isn't just physical preparation, it's mental preparation. You need to get to know how you deal with sitting on the bike all day and, sometimes, all night. You need to know how you deal with sleep deprivation. You need to know whether you have the mental strength to keep going rather than just giving up when you reach an inevitable low point. You need to learn how to listen to your body, when it's hungry and needs food (almost always during an Audax), when you're dozy, when you're thirsty, etc. Learning how you deal with stuff never stops, so don't assume a single SR series and you've got it licked.
d) Aim for at least an SR in 2013 in the run up to LEL.
If you can go for hillier rides (with AAA points) then even better. Lots of bits of LEL are flat but the Northern section is quite hilly (but worth it for the scenary).
Obviously, the more you can do in advance the better. Each training ride or each Audax means you'll suffer a little less on LEL itself (and hopefully not 'suffer' at all).
My LEL'09 preparation wasn't ideal, I was off traveling around South America with my wife for 2 months (Nov/Dec 2008) where I didn't go near a bike in all that time. But, in the 7 months leading up to LEL'09 I got my cycling fitness back and did a fair bit of riding (3000km of Audaxes and another 4000km of commuting before LEL itself). Before that I'd done an SR in 2008 and only got as far as a 300 in 2007. In 2006 I did my first ever 200s (two of them).