Transportation:: not sure where you're coming from, but if its north, park at Lindbergh and ride MARTA . This does 2 things: puts you against the flow before the race (you're headed north, everyone else is headed south) and after the race you can take either northbound train. Remember your Breeze card or fare.
Before the race: get there early and enjoy the crowd. They have lots of corrals, so if you're in corral ZZZZZ, 'early' is relative. It's different than a Disney Race, but its got its own type of energy. Lenox Square is ground zero. Sometimes they seem to restrict access to Lenox based on corral assignment, other times not. (If you are in a later corral, and do get access to Lenox, you can just camp out at Lenox and hop in your corral as it moves to the starting line, rather than hike to the back.)
For the race itself: it is HOT and HILLY, so pace yourself. It is easy to go out too fast on the downhill start and not have enough gas for Cardiac Hill. Enjoy the crowds. PRR has lots of spectators, with lots of energy!
After the race: if you're meeting someone, plan on which letter sign/balloon you're meeting at. Grab some of the Delta iced towels! Be prepared for a long walk back to MARTA. Wear that Peachtree shirt proudly! Make sure someone is assigned the task of firing up the grill for burgers/dogs/chicken/ribs/steak!