Old Mission Peninsula is pretty, Haven't biked it, but we have driven through the area. Montague to Ludington will likely have you riding some miles on the Hart Montague Trail. we rode that earlier this summer, it was a nice ride. The trail is a bit rough in places, but other than that it was fine. We rode it during hot weather and were glad to have the shaded sections. There are some details about the ride on our blog.
http://trikersbiking.blogspot.com/
50 miles is doable in one day, if you have been riding all summer, other wise it might be a good idea to stick with 30 per day. All the routes sound like they be great. Let us know which you chose and how you like it. Happy trails
TP