White 2012 Specialized Allez, Green 1996 Cannondale R500
There is a large Saturday morning ride that starts at the Descanso Gardens (Montrose/La Canada area) and winds south, and then east, and then back to the gardens. It's a pretty popular ride... I don't ride it, however my personal ride includes portions of their ride. Google the descanso gardens ride, I'm sure you'll find info.
If you want to spin in the late evening or at night, the Rose Bowl ring is there. On Tuesdays and Thursdays a very impressive peloton of riders that are far superior to me are there... to me, it's just awe-inspiring and provides motivation.
If you're in the shape for it, Azusa Canyon and Glendora Mountain Road are there, however both of those have far too much traffic for me to want to ride on.
The San Gabriel River Trail and the Rio Hondo River Trail both run through the SGV, however they are not favorites of mine. Poor air quality at times (proximity to the 605, lots of truck traffic maybe?) and incidents in the past of riders getting mugged on the trail... I have never had any problems.
I always run into groups riding through Sierra Madre and through South Pasadena, but not sure what their route is. South Pasadena at Trader Joe's I've seen groups gather more than once.. maybe there's a ride that starts there??
Sierra Madre PD evidently has a "zero-tolerance" stance towards cyclists, and will ticket you if you do not put a foot down at stop signs. A friend of mine is in the process of going to traffic school right now to keep it from ruining his insurance premium.
Good luck, and have fun riding in our beautiful SGV!