I was thinking about rando pacing during today's ride. Maybe my view is wrong? Maybe my memory is wrong. I know OP wants to do a fast PBP.
So, I looked at the average speeds of the PBP top riders over the years. They do maybe 32-35 kmh to Villaine and then the speed drops considerably after the honeymoon is over.
Myself, the three times to PBP, I did 5:53, 6:43, and 7:20 to villaines-la-juhel, which is the first control at around 220km although I think it was 231 km one year. The year I did 5:53 (actually less time because they made us wait, we got there too early), my typical 100km ride would indeed be just over 3 hours but that was decades and decades ago. I also think the French riders had pacers up there, it was a ***** trying to keep up. Two young guys with cat 1 legs with scars galore riding tempo forever. At 64 yo in 2023, I suppose I should be happy to break 8 hours to the first control or better yet, happy to still be riding pretty decent.
If I was OP, I would try to be able to complete a 400K on similar terrain as PBP and do it in around 17-18 hours. If so, you can almost get your sleep on PBP.