Interesting. If I read google maps correctly the SCT formally ends at the parking lot in Smyrna near the Creekside at Vinings condo complex and then converts to side walk bike paths along roads
as it meanders E/N for a few miles. A few yrs ago I was able to trace a southern bike path accessible on streets from the SCT at Cobb Drive that paralleled some major E-W limited access road but it does not pop up
on cursory exam now. That route was 5-10 miles long and ended a good ways south of Smyrna.
The formal start is on Mavell Road. The "sidewalk" path you're talking about is probably the spur of about a mile that heads up to the East West Connector and ends near the Publix parking lot. The existing, but unused, Silver Comet track is down a hill accessible from the spur trail, so continuing the official trail will be trivial from a continuity standpoint. The extension will pick up there, run under South Cobb Drive, and cross over Oakdale Road (the track is already paved over at the old Oakdale Road crossing) and continue roughly parallel to South Atlanta Road, crossing North Church Road and terminating at Plant Atkinson Road. I'll do a screenshot of the map and post it here later, but it's really a straight shot from the official trailhead at Mavell Road to Plant Atkinson Road, since all it is is the same Silver Comet rail line that the existing trail is built on.