Yeah it does take a lot of time to prepare for these events.. there's no shortcut to time in the saddle. Start by riding a few 200km events, then 300km, then 400, then 600, then 1200..
The ride itself allows 90 hours, which is divided into
- 40 hours to cover the first 600km, at a 15km/hr average (including breaks)
- 35 hours to cover the next 400km, at a 11.3km/hr average
- 15 hours to cover the remaining 200km, at 13.3km/hr average
(I don't think I have that exactly right, but it's definitely 600/400/200 split with a total of 90 hours.)
If you can ride at a reasonable moving speed and keep your feeding breaks reasonably short you can get a bit of sleep. This Sydney-Melbourne ride has a lot of climbing which means more time is going to be spent riding at the expense of sleep.
By comparison I rode a 1000km event in April; time limit is 75 hours and I used 66:30, and got 6 hours sleep twice. That was the easiest of these rides that I've done. The first two day was 360km (undulating then flat), second 330km (flat) and the final day 310km (but hilly).