I found a cool website that has maps of bike routes for ALL of Michigan. The quality of the maps varies.
http://www.lmb.org/pages/Resources/biking_in_MI/routes.htm
I've rode portions of some of these routes this past weekend unknowingly - Hines Drive and Huron River Drive - both are great!
There isn't one just for Huron River Drive... but a number of routes do include it. Go to the AABTS link.
Here is one: http://www.aabts.org/maps/dexter1.pdf
I rode Huron River Drive on Sunday. Very cool!