That route looks good. I would like to try as much of the road as possible, even the north section - it's something I've wanted to do ever since I first rode the nice parts of it and read more about it.Follow the left branch 10 km to Orrville, keeping right at the two intersections you encounter on the way. This takes you over the Turtle Lake road, and on to Star Lake Road.
Also, last weekend as we rode along both the 141 and the 518, I took a look down Turtle Lake Rd as I rode past it a few times and it looks very doable for our bikes on street tires. Turtle Lake Rd is hard-packed gravel.
Once on the 518 and about to head north on the Nipissing Road, we can also go right (south) and try out the part of it that seems to lead to a dead end. It might be too rough or tight for big bikes, but I'm curious. (Coordinates: 45.423511, -79.676472)