As a rough guide, I'd suggest that you would want to be able to comfortably do a minimum of 1/3 of your target distance every time you go out for a longer ride, and work up from that base level. When I was building up to "Ironbum" (a self-imposed extension which brought the total ride distance up to 320km, for those newer to the Forum), my base level was 100km. From there, I went through a four-week cycle, adding a bit the next time around, then the third week dropping back a bit, but not all the way back. Then I'd go further again for the fourth week. I guess you could call it a two steps forward, one step back approach, with the steps in each cycle getting progressively bigger. As the miles really added up, I took it out to a six-week pattern, but with the same approach. The final big ride was a 280km, solo jaunt which included a loop through the Strzelecki Ranges out the other side of Westernport - although 40km shorter than the ultimate goal, it was a much tougher ride, which I coped with remarkably well.
Regardless of which distance your target is, this approach will work, and you do not need to achieve the full distance in your training before the event. If you can get to about 80%, you'll be fine.
And the "on-board" kit:
In the saddle bag, a spare tube, patch kit, tyre levers & Allen keys to suit the various bolts on the bike.
In the pockets; phone, ziplock bag with some cash, card & some ID. Change of glasses, 'cos it was dark when I left home, ziplock bags of Gatorade powder, measured for drink-bottles, and a bar or gel for every hour on the bike - anticipating up to 13 hours.
And I had four bottles on the bike.
Just ride the bloody thing and enjoy yourself!