We ride a bagger trip across the UP every year and day #1 is always Mercer WI to Munising, so this is first hand information.
The most direct route is Winchester WI -> Presque Isle -> Marinesco -> Watersmeet -> Sidnaw -> Three Lakes -> Ishpeming -> Marquette. This is 200 miles +/- 10 miles and keeps you in the LES zones most of the time. Last year it took us 7 hours to ride from Mercer to Marquette including a 1 hour stop for breakfast in Watersmeet.
There is another route that will take a bit longer and depend upon good snow conditions. You would still ride to Watersmeet but then continue onto Iron River -> Amasa -> Republic -> Ishpeming -> Marquette. The trail between Amasa and Republic is only about 3 years old and not on most maps.
Either way, I would suggest breakfast out on the trail at the petrol station in Watersmeet. It is about 60 miles into your journey and gives you a nice break. The restaurant next door to the station serves a good breakfast.
