Open up Google maps and add the cycling overlay. If you can get to some of our bike lanes and paths (Beltline, PATH trails, etc) then you're about as good as it gets in ATL. With some creative route building you can do 50-60 in town and get in a decent amount of dirt in the process.

ITP generally- stay off the big roads and you're mostly good here depending on time of day. There are loads of ways to string a route together if you don't mind lots of turns and climbing.

OTP generally- YMMV but I hate riding north of town unless you're like 30-40 min north of the city. It's just too busy and I've never been able to find roads that didn't have me looking over my shoulder all day. There are some established routes to the east and south of town (check ridewithgps) but IMHO the good stuff is west of town. Once you get past most of the development, there are some killer quiet country roads all the way to the AL border.