Another thought on mental - not so much positive mental attitude but being in the right headspace. My first PBP (2007) was ridden in shocking weather. It was twice the distance I'd ever ridden before. My saddle broke at 4am 15km out of Brest and I had to ride the next 65km either in a BMX position or out of the saddle, before getting a new saddle with 600km to go. Yet there was never a point where I thought I wasn't going to finish.
Second PBP 2011, way more experienced, wanting to do a fast time, stressful experience at registration where they decided they didn't like my bike. Suffered and was miserable all the way to Brest and on the point of giving up, before reducing the level of giving up to. "I'm stopping to sleep in Brest and to hell with the time." After that, planned to tour back without a care in the world and rode an indecently fast 400 back to Villaines and ended up finishing less sleep deprived that an the start.
Third PBP 2015, way more sensible. Set off with the objective of enjoying it, towed a group from Mortagne to Villaines for the sheer hell of it, and generally had a ball.
I've been successful at Audax when I've managed not to worry about it.