My first (only) 210 ATB was done on the back of training rides of about 100km each over relatively flat roads.
I think the big consideration, in terms of ATB in particular, is the speed you can sustain on the route. You need to consider that the total time allocated is 12 hours and, with the ferry option, you lose at least 90 minutes with marshalling, loading and crossing. That leaves you 10h30mins riding time max. Add to the mix the fact that there are about 4500 riders going the same circuit as you and the ferry carries 650 per hour (plus whatever extras BV can charter) and you can see that you need to knock off the trip TO the ferry pretty smartly or you'll be waiting a while before you board and that's time out of your return leg.
In my case, my ride partner suffered from arthritis and a quick dose of the runs en-route to Queenscliff. We didn't sail until 2:30 which meant starting for home around 3:30 leaving a nominal 2 hours to complete the 100km from Sorrento. I finished late and my mate caught the sag wagon and train.
BV have done a lot of work to try to improve the ferry logistics but, in the end, when you add it up there is only so much capacity and if people don't ride fast enough to fill the early boats, there's always going to be a backlog.
Of course you always have either of the down and back options which gives you a full six hours to complete each direction.
That being said, I'm hoping to hit the 210 again this year and do it in rather better time so I do enjoy the finish celebrations.