Depending on when you're in ATL, the walkway between Concourse E and Concourse F will be nearly empty and a good place to run. You could combine this with the underground tunnel from T to E. I've walked to every gate in ATL on a 3+ hour layover before and it's a nice tour of the airport.
Alternatively you could do as was suggested in an earlier thread and visit every SkyClub in ATL.