I put together a new route based on what JLW posted. I've been up in that area a lot, but don't know many of the back roads. I tried to put together an interesting route back from Mackinaw City... someone want to check it out and chime in with suggestions?
2008 Tunnel of Trees Route
Total time is listed as 6 1/2 hours for the entire loop. So, leaving at 10:30, that 5 PM without stops... some time for lunch, and Leg's Inn maybe 7 or so back in town I'm thinking.
For anyone wanting to get back from Mackinaw quicker, a trip down 31 or using 131 would provide for that as well. If you follow 131 south you can still pick up Supply Road and come in the back roads into Traverse City. I've done Supply Road in the past, but not in the Solstice.. should be fun to drive, and tons of trees lining the roadway.