loaded question.

bike setup depends on the course more often than not. Lots of marathon/endurance riders are on FS to soften all that saddle time but hardtail works too, hell some guys even go fully rigid SS. What kind of 24 hours are you looking for?...fun, challenging, brutal?

I can tell you nutrition is key, make sure you have that dialed before race day. Go on a few 'day trip' rides if you can. 6-8 hours and practice managing food/fluids to avoid that dreaded bonk.