Surly LHT; Surly CC (as fixed-gear commuter); Hunter CX; Dahon Mu Uno
I did some biking in the UP in mid-May back in 2007. I rode up the western side of Lake Michigan, spent some time on the UP, took a ferry to Isle Royale, and then headed home down the eastern side of Lake Michigan. Some of my route overlapped with sections of what you're talking about.
On the way up I followed 2 westward from Norway (a little east of Iron Mountain) to Watersmeet. I then took 45 northward from Watersmeet to Rockland, at which point I followed 26 out onto the Keweenah Peninsula. On the way back I did a lot of my west to east travel on 28, but got down to 2 by Naubinway, and then stayed on 2 all the way to 75/ the Mackinac Bridge.
I found the sections of 2 and 45 I rode to be totally fine. There wasn't much traffic, yet there were decent shoulders. I was pleasantly surprised.
The western portion of the UP was hillier than I expected, but the eastern part was pretty flat. The climbs weren't super long, but there was a lot of up and down which I found to be fairly tiring. Also, I couldn't believe the easterly winds I experienced while heading east... I'm not sure how uncommon that is, but it was a killer to face that wind several days in a row.