I've lived in St. Paul for 35 years and I have a few suggestions:
1. There are easily 250-300 miles of excellent bicycling you could do within the Twin Cities metro area.
2. You could take Amtrack from St. Paul to North Dakota and bike back from there on the Northern Tier.
3. You could bike to Milwaukee, LaCrosse, or Columbus (near Madison) and take Amtrack back from there.
4. Twin Cities to Duluth and back would be about 300 miles. I believe you can do most if not all on bicycle path.
5. Wisconsin is great for bicycling because there are a lot of small towns and facilities along the way. I did LaCrosse to Milwaukee years ago and it was very nice.
6. There are a lot of converted rail beds (
http://www.railstotrails.org/index.html) all over the place, particularly in the Sparta-Elroy area of Wisconsin but also throughout Minnesota. I believe there are reasonable Amtrack connections to these areas.
7. I've bicycled across the northern part of the state (state highway 1) and found it quite enjoyable. Little traffic but not as many small towns, hotels, restaurants, etc.
8. There are a lot of nice trails in the Milwaukee and Madison areas.
9. You might get lucky and be here when they have RAGGBRAI, an organized ride across Iowa with thousands of participants. I believe there is also a smaller version of this in Minnesota.