I had a look into this and in the end didn't bother.
I have done loads of routs here along with a coast to coast ride but the planning for L2P just seemed so much hard work, the two things that put me off were, 1- i have spent a lot of time in Northern France on small minor roads and the signing can be somewhat hit and miss, getting lost almost everytime i have been over there 2- entering Paris is famous for being a major problem, as most of the roads going in a major roads and to busy to cycle, so its in through the suburbs and have heard many stories of getting lost in the burbs.
All the best , i decided to go on an organized trip but admire them who go for it and do it and succeed .