If you are going to the afternoon session, then take a bus to the Magic Kingdom, then take the monorail to the TTC. When you get off the monorail walk over to the Kennel/Lost and Found. There is a Richard Petty van there that will transport you to the racetrack.
If you are going early in the morning, you generally have to be there about 7:30. I would take a cab. The earliest breakfast bus to the MK would be about 6:30, then you would have to use the same steps above to get over to Richard Petty. That is just cutting it too close.