Magic Kingdom buses will start running 45 minutes prior to the earliest MK Area Character Breakfast seating time. When boarding the bus, inform the driver as to where you need to go. If the driver is able to, they might be able to take you directly to the Grand Floridian. If they are unable to, they will direct you to the Monorail which will take you there. For an 8:20 PS, I'd be out at the bus stop no later than 7:45.