There are two parts to training for an event like LEL. The first is training the body to optimise its performance and those above are far more qualified to comment about FTP, VO2max et.
The other bit is about training the head and the gut to deal with 4 days in the saddle, without time to properly digest food, and dealing with weather and hills when you are at a psychological low. You may be lucky and find the answer to these challenges straight away but you may find it helps to practice - with some training rides at unusual hours, in the dark, in bad weather, find routes where you are forced over that &*$%* hill at the end when your legs are falling off, or where you have hours of headwinds on the way back. (Not all the time, I hasten to add, that will take all the fun out of cycling that the turbo-trainer hasn't, but so that you have the experience of dealing with tough situations out on the road).