I would get some 600k's finished before I even thought about a grand randonnée. Much less traveling to France for my first grand randonnée. Of course, pbp was my first 1200k, but I'm here to tell you it's a bad idea. And that was my 4th season and I had finished 4 SR series. It would have gone so much better had I ridden a 1000 or 1200 in the U.S. because most of those rides have a lot more structure and operating on automatic is very important when you have been riding for a couple days straight.
If someone is very fast I would have fewer concerns.