Yetis, you are south of me I am between la Crosse and Prairie Du Chine along the Mississippi river. Lots of good riding in the area also rails to trails can take you a long ways in Wi and many interconnect. Although it is doable the cold and wet weather and wind may get to you. Since you are in Janesville area you might hook onto the Great River route south and either catch part of the T A going west or continue to the southern tier as some one else suggested.you could ride to La Crosse hop the train and head down the west coast route lots of options have you looked at the maps fro Adventure Cycling yet? if not check their web site.
All good wishes
CF

MS you are right Montana will be pretty cold at night I went through mid to late September. October would be worse Some mornings the tent was covered with frost took extra time to dry things and get going. The worst is getting out of the tent middle of the night for nature call uo in northern Wisconsin I woke up to a snow storm had to hole up a couple days. The weather did improve and the nights got colder