I am not a hard core cyclist so take this with a grain of salt ... perhaps consider riding east on Hwy 2 over to Ashland then heading South on Hwy 13, that will take you most of the way there, with some small jiggering once you get close to Madison.

The Wisconsin Bicycle Maps. 
You can order them here:
http://www.bfw.org/store/index.php

There is a trail that starts in Allouez by those docks that goes at least to Iron River.  It is definitely gravel and there are ATVs but the trail is pretty wide.  Start there by the bridge to the Meteor and the old Boathouse restaurant.

Carla--I believe that would be the Tri-County Corridor, which runs from Superior to Ashland.  You can actually get on it by Barker's Island (the trail has a different name in town, which is escaping me at the moment).

I recently rode to LaCrosse on the Hwy 35, and it was beautiful.  The Mississippi river valley is great.  I have taken the bike trail from LaCrosse to Madison before and recommend it.  I would go with Google's bike directions.  Have fun.

I biked from saint paul to Duluth when I was about twenty.  I have a few suggestions.
1- wear sun screen.  The sun can burn your skin.
2- have some cash with you.
3- bike many miles a week in the months prior to your trip.  Hundreds.
4- don't drop acid at the start of your ride,