I hope you get some first-hand suggestions but Google gives at least interesting
results. I asked it for directions from Grand Rapids, MI to Manistee, MI, clicked the 'bike' icon and it gave me three choices, two quite similar on White Pine Trail
(130 miles) and one different on Musketawa Trail
(125 mi). The trail sections for both routes are quite short and the rest is on many back roads with lots of turns; Google's long cue sheet is very detailed!
Are you doing that ride in the winter? That would be impressive, and maybe some other adjectives!