You could easily spend two weeks traveling up and back along the Mississippi River on the Great River Road trail between Missouri and Illinois without having to ride on the same side of the river twice. I wouldn't go further north than Quincy, IL if you want to stay in the land of Twain but could ride down to the confluence of the Ohio and Mississippi rivers. Lots of paved bike ways to hook into like the Sam Vadalabene paved portion and other network of trails around St. Louis and Madison area. There's a nice series of maps that is all drawn out for you but I can't find the link at the moment. Can work on it for you if interested. Btw, lots of points of interests between Twain and Rogers and Clark in that area.