The southern trailhead is behind Arhaus furniture, across the parking lot from BGI. The trails really do not link up with the ones behind Autumn Woods anymore due to the floods this past year. But they do come out behind River Bend Apartments, right by the tennis court.
BGI had Spectrum Trail Designs put in a new half mile loop last year, and there is another loop off that trail that adds another half mile. Beyond that, there's a few other trails, but really just one main one that goes underneath 465 to to River Bend. Riding the trails up the east side of the river, then roads to TRTP, a loop of TRTP, and back is around 13 miles.
The trails are beginner friendly, and need more tires on them! I plan on doing some more work to these trails later this fall and winter. While there is no recreational easement, the trails are ok to ride.
Send me a PM if anyone has questions. I work at BGI and look out my window at these trails every day.