It is a VERY hilly place – even so, 12hrs is playing it safe.
I've done 80km there and it probably took nearly 8hrs in all (and lots of will power), so I reckon 100km would take 10-11 hrs + breaks.
radoggair – doesn't your average speed drop the longer the ride – mine does.
Do 100km – you can always bail out & stay on the 75km route if needed.