Good suggestions, Carol.
A lot of it depends on what day of the week you are driving through Atlanta. On a weekday, you might hit heavy traffic. On a Saturday or Sunday, the traffic isn't so much of an issue.
We have been known to leave from the north side of Atlanta late in the afternoon on a weekday, or even after work -- and we can easily get to Valdosta (about 4 hrs) or even further sometime that evening, even if there is heavy traffic in town. If you don't mind driving late, you could even go as far as Ocala (about 6 hrs). You could then be at the parks pretty early the next day. We do plan ahead and make a hotel reservation for where we want to stop. If we leave in the morning, though, we will drive the entire way.