Not too many great routes that I've ever used. Generally Sierra Madre to the bike path over to Encanto park, then down Royal Oak, up to Foothill at Mountain. I've usually taken Foothill to Walnut, then down to Colorado to get from Pasadena to Glendale.
Fine route in the morning, less nice in the afternoon.
The other alternative I've used is turning left off of Foothill on Santa Anita, following Huntington around the track, then turning right onto Sunset, which turns into California. More casual ride than Foothill and Walnut once you get there. You still have to hook up to Colorado to get from Pasadena to Glendale, though.
I believe there are ways to get to Glendale through the canyon roads around Scholl Canyon, etc. but that would add a pretty good dose of climbing and take much longer. Never done it, so someone else could chime in on that.
If ignorance is bliss, shouldn't you be happy?