I don't know if I'm too late on this one, but you could easily accomplish your goal on some really great trails. Hope over one of the bridges to Duluth and hop over to London Road...which then brings you to Hwy 61. Follow that (it's the same route as Grandma's Marathon) on out; it should at some point parallel the major bike trail up north...I think it's the Gitchi-Gami. Anyway, it's hard to miss because it's paved, relatively new, and follows alongside the highway. You can take that north all the way to Grand Marais.
OR...
you can hop over the bridge, again into Duluth, but this time head south. You'll have to take a few backroads (most of which are marked with arrows directing you to "trail") and head to the Willard Munger Trail (Munger Trail)...which you can then take all the way to Jay Cooke State Park and even past that..turn around at your half way mark and back to Superior.
MN has the most connected bike system of any state in the country. Once you get into our state and out of Superior, you can easily do 100 miles.
"There is nothing more frightening than ignorance in action." -X