I doubt I'd be able to do 160 km - furthest ever at the moment is 103 km in a day. I'll be doing my furthest ever in a day on ATB this year - approx 120 km (Rosebud -> Sorrento -> Melbourne).
Its a pity we can't register for the "Slog to Sale" after the Sorrento -> Melbourne!
(Besides ... Saturday 28th October is just before my final two biostats assignments are due in, one due 1st November, other due 6th November - then done until end of February next year).
But, for your own information, I looked up the Pakenham train timetable ...
There is a train (first train on Sat morning) that arrives in Pakenham at 6:35 am - not sure which train station along the Pakenham line would be easiest for you to get to at *that* hour of the morning!
Dep Flinders St 5:28 am
Dep Caulfield 5:44 am
Arr Pakenham at 6:35 am
You'd have to see how far it is from Pakenham station to the start in Pakenham though.
Hope that helps a bit.