The only flat course I can think of in Atlanta with markers would be the Silver Comet Trail over in Smyrna. 35+ miles of rails to trails paved path with limited road crossings. The first 4-5 miles can be semi-busy, depending on time of day, but it's dead after that. The markers are really weird, in that there is no rhyme nor reason for their regularity. There are mile markers spray-painted on the pavement though, that are very accurate. The "official" markers go 0.0, 0.3, 0.8, 0.9, 1.2, 1.6, 2.4, 2.9, 3.2, 3.3, 3.7, 4.2, and on and on.
This trail is SUPER flat. To get there, take 285 west to S. Atlanta Rd. exit, go north, left on East/West COnnector, right on S. Cobb Dr., left on Cooper Lake, left on Mavell Rd. and then park at the end of the road (big parking lot) and this will be the 0.0 marker.
When I used to live near it, I would go on a Friday morning at like 4am for my 20 milers and even in the pitch dark you could still faintly see the divider line painted on the pavement - it was so peaceful and relaxing just cutting through the darkness. The pavement is perfect, so no worries about footing, either.