I'd say an SR series is a darned good idea too.... and also rest. Don't be afraid to take time off to let the miles soak into your legs (thanks to Ian H for that expression!)
Also, consider consecutive days in the saddle. This is something that it seems not many people do, so maybe it is not necessary, but I personally feel worthwhile. Do a 100km one afternoon and 100 the following morning for example. Perhaps not necessary from a physical point of view but helps on the mental side if the legs are feeling a little tired. You get to learn that tired legs work so long as they're fuelled!
It's worth noting that there's a level of motivation that kicks in once you're on LEL (or any such ride for that matter). You're only there to ride and there's a buzz about it all - and this can make up for any shortfall in the physical preparation. I wouldn't plan for it but it's worth noting to ease party fears.
To my mind, success on such events is also about a kind of what I'll call an ad-hoc planning. That is, don't go in with any concrete notion about how you're going to ride it, such plans can crumble very quickly and could leave you despondent. Better to remain flexible and prepared to rethink on the fly. Things change so be prepared to revise plans according to prevailing conditions. Obviously, experience plays a big part in this so don't expect yourself to know it all.
If it's your first such event, you're going to learn a great deal about yourself but, sadly, nobody can advise you exactly what you'll learn. So, with it being a learning experience, be prepared to make 'mistakes' and therefore cut yourself some slack in advance. There'll be things you'll do differently next time around! I know that was my experience.
LEL in 2009 was definitely a learning experience for me. Not just the ride but also in my preparation. Even though I was as well prepared as I could be. I finished in time, so it was a success in those terms, but I faired better in PBP this year (MUCH better in fact!) and I'd say that one of the reasons for that was purely the experience of LEL.
Whatever happens, remember it's only a bike ride. Not the end of the world if you don't finish. Main thing is to try and enjoy it... well, as much as is possible anyway!