May I suggest, take I-69 to Indianapolis and then take I-65 all the way to Montgomery, AL. Take I-85 east to exit 9 ( Route 271 ) south, about 5 miles, to US-231. This is a four lane 65mph road that passes round a couple of big towns but does not really slow you down too much. There is a Flying J on the south side of Dothan, AL that is a good place to stop and eat and get your last tank of cheaper fuel before Florida. Then continue south to I-10. Take that east to exit 225 ( US-19 ). US-19 is another four lane 65mph road that passes through some small towns. This is the favorite part of the trip, for me. There is almost no traffic and it goes through some beautiful Florida back country. Go south to Chiefland, FL where you will take ALT US-27 south. This will bring you back to I-75. From there it is on to the Florida Turnpike and then the Western Way Tollroad ( 429 ) and the WDW western entrance.
This is only a little longer then I-75 through Atlanta, but I despise driving through Atlanta and that goes for I-75, too. We have done this route in our 38' motorhome the last two trips and there are no mountains. Other then Nashville, there are no major cities to go through either. I've always taken I-65 to I-24 to I-75 when going to Florida, but I have found taking the above route I arrive more relaxed and enjoy the trip more.
Someone mention getting from Traverse City to Atlanta in 15 hours, heck, it took us 14 hours from Jackson.
We always try to stop before 9pm as we have found motels booked up sometimes by then.