Dauset Trails Nature Center, Exit 201, 360 Mt. Vernon Ch. Rd, Jackson, GA 30233
Free. User Submitted so I can't guarantee it's right off the road, but it looks like it is. Only open Tue-Sun 9-5.
30 miles of trails.
The Thread, Exit 18, La Grange, near Alabama border, 4 miles off the highway
Free. This is a great walking, running, biking trail that winds through the town and neighborhoods with at least 13 miles. Neat little downtown, restaurants, gas stations, the works. Cool little bike shop called Goldens on the square. There are a lot of access points for the trail. The map below shows parking as well as a trail map. I got the T-shirt below at Goldens.