400 k or so is a good distance. I would definitely work up to it, brevet style, and do a few 200ks, then a 300k about a month before the main event. When I did my 600k, I had no clue what I was doing, but it went fine. When I did my 300 and 400k a year or so later I was a lot better prepared. Anyway, here are some things that worked and didn’t work for me on my 600k:

Great planning:

Extra pair of shorts to change into and put the old ones on over the new ones at the 100 mile to go mark
The hotel room at the 100 mile to go mark instead of the out and back half way point – good timing
Hooked up with some people for the 10 pm to 4 am stretch. May be more of a concern for a woman than a man (it was a sketchy place from a safety perspective), but the company in those hours was welcome regardless.
A small roll of duct tape allowed me to patch unexpected hole in shorts.
One of those astronaut foil emergency blankets (I am a backpacker at heart)
Ride to the next controle. Period.

Not so great:

Brought too much food – literally POUNDS too much food.
Way too many tubes and CO2s brought, but this was a completely unsupported ride with no bike shops at the controles or on the route – you never know. I’d probably do that again.
Tested a light system for the first time that did NOT work (the mizuno spacebar which broke and almost cost me my $500 Nite rider moab). Fortunately brought back up helmet mount for said light. Aces.

More points:

- Don't underestimate the last 100 miles. The last part of the ride was all "home territory" for me, and I never thought it would take me 11 hours to ride 110 miles, but it did!

- Things that hurt now may not hurt later. Keep riding.

- It is ALMOST all mental. Your body can keep going (granted not at its highest level) for a very long time. It's your mind that it going to talk you out of or into it.

- What you like to eat at mile 150 may make you throw up at mile 210.