Here's a few I have put together on Bikely (haven't had a chance to ride them all as yet).
(107km) http://www.bikely.com/maps/bike-path/10 ... marsh-loop
(131km) http://www.bikely.com/maps/bike-path/cs ... erribee-cs
(105km) http://www.bikely.com/maps/bike-path/10 ... ara-return
(82km) http://www.bikely.com/maps/bike-path/ta ... and-return
(80km) http://www.bikely.com/maps/bike-path/80 ... a-and-home
I also often ride this (30-40km loop) and occassionally ride it 2-3 times for a 70-100km ride.http://www.bikely.com/maps/bike-path/32 ... lakes-loop
You also have the option of following the Deer Park trail onto Federation trail that ends at Millers Rd in Altona.
From there you can either ride down to Williamstown and along the bay trail or follow Francis St and into the city (which is the way I ride to work from Taylors Hill/Caroline Springs). From CS to Millers Rd is about 24km, so there and back is close enough to 50km's.
If you're ever looking for someone to ride with, send me a PM and if I'm free I'm happy to tag along.
Also, the Melton Cycling Club have regular rides each weekend (which I am a member of), so a ride to Melton is 20km's and one of their group rides may be 30km's so that ends up a nice 60-70km ride....just another option.
There is also a group of mainly Filipino riders (called Caroline Springs Macca Riders - on Facebook), who ride each Saturday and Sunday from the Maccas in CS.
They are a fairly full-on group but a really nice bunch of guys and often ride 70-100km's on Saturday's and then a shorter recovery ride on the Sunday.
There's plenty to choose from, it just depends on what type of riding you want to do and what direction the wind is blowing (and whether or not you want to battle a headwind or not)...as well as if you want to ride by yourself or with a group.
Hope this helps....