SGRT terminates at Seal Beach but it starts way up by Azusa... not really local for you. it's not clear from your post if you plan on driving to your take-off point or not.

If you really want to start down town, the LA River Ride might be a good route to emulate - here's a link I found (this isn't me but it looks about right)

http://app.strava.com/activities/63948706