I rode at a place while I was there only about 1 mile from route 400 (I think?) It's the highway that goes straight up from midtown through Alpharetta. That's probably your best chance of finding a nice place to ride that you can get to by public transportation. Atlanta is very spread out. The mass transit system there doesn't go very far out. I would google the Alpharetta area for starters. There's also the Horse Park they used for the 96 Olympics, but that's way out in Conyers, so you'd need a car. Good luck.
You just have to see your distance...you don't have to like it.