Travel on the inland waters is limited to daylight due to need to avoid debris that won't show up on radar. Also, recreational boats are lowest priority for locks. It is not unusual for rec. boats to wait several hours for a lockage, which itself will take close to an hour. So, you can't assume that you'll get 100 miles in a 10 hour day. On sections of the Illinois waterway, and for most of the Tennessee-Tombigbee Waterway, there will be multiple lockages within a hundred mile stretch. Add to that the fact that most fueling locations are 9-5 operations. Plus, changes in water levels will impact your travel. In July and August, the Illinois was closed for weeks due to high water. I estimate my travel at 50-70 miles a day, depending on the area I'm in, plus a lay day every 3rd or 4th day to account for lockage delays, fueling and weather.
I'd avoid the lower Mississippi. It is unlike any other water that you'll boat in. It boils and swirls, has a vicious and unpredictable current, and is full of debris. The tows on the lower Miss are the largest vessels on earth and sometimes require the whole width of the river to maneuver. And, except for Memphis, Greenwood, and Baton Rouge, there are no facilities that cater to rec. boats. Anchorages are unpredictable due to rapid changes in water levels. While a straight open river passage looks inviting, it is more dangerous than you can imagine.
Low bridge on the entire trip is the BN bridge south of Chicago - figure 19' depending on the water level.
My last trip from Chicago to Paducah took 2 weeks due to high water from a hurricane that hit the gulf 2 weeks before. Paducah to Mobile, with good weather and minimal lock delays, figure 10 days to 2 weeks.