if you want to ride lel and enjoy it (i.e. not have aches and pains, have enough time for proper sleep), you need to be quite* fit. three years is plenty of time to prepare, one year would be enough if you are "normally fit" (not too overweight and in good health). for unprepared it is a very hard ride, as it was in my case..
i've learnt a lesson and prepared for this year's big ride 1001Miglia and didn't have any problems and enjoyed it thoroughly. it always helps to be faster as you can be more flexible with your time.
*to ride 200km in <8hrs or ~500km in 24hrs in flatish terrain
Before LEL my fastest (and unique) 200 was 10 hours and I could do a 300 in 15 hours. I think that the 300 time is the most relevant since one tend to do 300 and a bit days. I finished LEL with enough time in hand
to stop worrying about time when I was in Yorkshire on the way back.
Ride one sub 15 hours 300 and you will be fine!