Hattiesburg is the eastern terminus of Mississippi's finest paved bike trail, the Longleaf Trace. You can do 40+ miles one way, to the other end at Prentiss (where I live) and there are lots of small county roads with some hills if you find the trace too boring. Also lots of nice hilly backroad loops around the Prentiss end, I can provide more details if you're interested. The Longleaf Trace passes through southern woodlands and farm country, small towns, etc. Practically empty of users on weekdays.