Audax Australia runs a number of rides from 50km-200km+ in the week leading up to the ride. You can register for these rides via their online ride calendar.
The following websites have some helpful tips on training:
Cycling Inform – Download a free Alpine Classic Training Program
If you’re contemplating taking on the ACE250, here are more tips that might help: from a guest post on “The Climbing Cyclist” by Anthony Gugel – http://theclimbingcyclist.com/guest-post-2012-audax-alpine-classic-250km-version/