Thanks to Fred Camino, who has put together an extensive post on how to take Metro buses to and from this year's event.
My advice: Either plan with a friend and get to Venice Beach around 8 a.m., parking one car, then carpooling to USC (where you'll park the other car).
OR: Park at USC, walk to Venice, then eventually take Metro back to USC.
OR: Park at Venice Beach at 7 a.m., allowing yourself enough time to board a Metro bus to USC. That way, your car is waiting for you when you arrive.
We've tried and failed in the past to drum up enough interest to rent a bus to transport people back to the start line. But if any of you would like to take the lead, I can publicize your plans and hopefully get enough participants for you.
Go Metro to The Great Los Angeles Walk 2009 (Metro's The Source blog)
Metro Trip Planner