Unless you are a really strong rider, sleep is comfort and anything that takes time from sleep is discomfort.

Many RUSA 600K Brevets are far less hilly than the 615K to Brest. Many Americans do not ride much in cold, wet conditions. These two factors combined with a strong quartering wind that provides little protection makes for a more challenging situation than we have back home.

Riders that struggle to finish US based 600K Brevets in say 35 hours will be challenged unless it was the Eastern PA 600K or some others that are truly very difficult. ACP publishes data that I believe supports my above contention. Nick is a solid rider and I remember reading an account where he had his PBP planned down to the minute with the exception of Pneumonia that took him out at Dreux. I seem to remember that his analysis gave him like 2 hours to spare. His spreadsheet was amazing. On the other hand, if the OP can easily do sub 30 hour 600K Brevets, bring the slippers; however, rider statistics show that most Randos just make the time cutoffs although many do so by choice. It is worth mentioning that we should be careful if we compare our RUSA Brevet times against other riders simply because we do not know the conditions of that particular Brevet nor do we know how the rider chose to ride it. A rider might do a 35 hour 600K by choice and the same rider might do a sub 24 hour 600K by choice on another day or another course. Each person's experience will be unique, the cold water you hear is a caution not to waste time on the ride. I did a 600K with a multiple Charly Miller type rider and he took tons of time eating and sleeping en route to about a 36 hour 600K. He could afford the comfy sleeping and eating because he was so fast. Most riders do not have that luxury. Without knowing anything about your randonneuring history and I doubt any of us has seen someone carry spare footwear, you got solid advice. I would carry a space blanket before lots of other stuff because at least it will keep the dung off you and kind of dry when you lay down to sleep in 45 degree temps with rain falling.

I seem to also remember West Point Grad and maybe a Ranger who had been to war who claimed PBP was the hardest thing he ever did.

The numb fingers and feet sometimes return to normal.