SGRT is really far east-- it's a long ride east through less than great neighborhoods from the airport to get there, then another 12 miles or so back to pasadena from Duarte. Going up the coast and over through hollywood would probably be better.
Another route from downtown that probably isn't bad is North Broadway to Huntington to Fair Oaks. I've biked from the Monterey Hills area (~Monterey and Ave 60 or so) to Pasadena, but have only driven the broadway part.