My wife and I will be riding the Alpine so I may be interested in joining you for a few rides.
We've been 'using' cycle touring to keep our base level fitness up, and if you manage to squeeze in a tour before the event it should help a lot. We both found the ATB to be pretty easy, largely due to cycle tour conditioning.
I strongly believe that riding long days loaded up with luggage (in hills/headwind) has improved my conditioning far more than when I was actively 'training'.
We live in the Northern suburbs, but riding down to the peninsula shouldn't be too much trouble.
We usually ride up north around Sunbury - Macedon area, if you ever want to ride up here.