Les Diagonales de France Just start from Dunkerque and do three Diagonales de France to finish at Dunkerque. Ferry both ways.
This is known as a "triangle" in French circles.
It earns you lot of respect, people tend to do it in 2 weeks though.
If you are interested I can put you in touch with people who can tell you how to enter. It works more or less like a DIY except that there will be people along the way riding a bit with you to cheer you up, they will also ask you to produce a road book (effectively a list of roads you intend to take). You need to enter long before riding it.
If you finish, you will then be part of the "diagonaliste" fellowship.