This is a well-marked point-to-point trail on the Mason Tract property (managed by the Michigan DNR). The terminal trailheads are at the Chase Bridge Road crossing over the South Branch Au Sable River at the south end, and along M-72 on the north side. There are also sand and gravel roads paralleling the Mason Tract Pathway that offer access points along the route. This trail closely follows the South Branch Au Sable River and is popular with anglers, so don't be surprised if you encounter men and women with chest waders and fly rods. In fact, the property is named after George Mason, one of the founding members of Trout Unlimited.
This FKT route is a point-to-point between the Chase Bridge Road and M-72 trailhead parking lots, following the main trail and excluding the Thayer Creek Loop section. The trail is well marked singletrack through mixed forest, with some mild ups and downs. The footing is mostly sand or packed dirt, although there are some wooden boardwalks, tree roots, and a few areas that can get wet and muddy (especially after spring snow melt). This route could also be completed as an out-and-back.
The Michigan DNR map of the trail can be viewed here: https://www2.dnr.state.mi.us/publications/pdfs/RecreationCamping/Mason_Tract_Pathway.pdf