Wish I was back in MN so I could train with you for my own century.
I'm not sure where you are in the TC's but I was based in St. Paul and so my ride sort of revolves around the river.
A nice starter loops is just around the Mississippi river. I would do loops of these for more distances and because it is easy to make it shorter if you need to. there are several bridges you can cross: the ford bridge, lakestreet bridge, franklin, and others further north but I'm not sure of the names. since the river is on one side and depending on the side of the road you ride on, traffic will not intercept you. the traffic tends to be pretty low anyways. I think this is a supersafe route.
the grand rounds extends that to more of minneapolis.
http://www.byways.org/explore/byways..._n_go_2243.pdf -
there is another route I really like that also follows the river. I only did the shorter 30 mile distances mostly due to traffic. this follows a lot of the mississippi river on the south and west side of st. paul. it is on a bike trail but I usually switch to road because part of the trail was really broken up with a lot of debris- judge for yourself. or maybe they fixed it.
http://www.bikeclassic.org/event/ind...ion=course_map
you can also bike the gateway trail for a real long ride.
m