Just go down I75 until you get into Florida and you will see the turn off to go to Disney North of Tampa. I75 is smooth highway all the way now (didn't used to be that way)and there is only one semi hard climb just as you enter Tennessee (Jellico). Just drop down a gear and follow the trucks if necessary. The rest of the way is mostly small hills. The trucks are heavy in places as you near a large city but nothing really bad.
We used to go down via I64/I65 because of fewer trucks but the road is much rougher and the citys harder to get through or around. Have found I75 to be our choice for the past 8 or 9 years. There are plenty of campgrounds and gas stations available also. You are already past the worst part of the trip (construction from Toledo to past Dayton). We usually try to go straight through Atlanta around 10:00am or a bit later. Never had a problem that way. Also, try to drive one lane over from the right lane so you don't get caught in the many lane endings and people trying to merge. Most of the road through Atlanta is multi lane so you will not be holding anybody up by doing that.
Hope you have a smooth, pleasant, and stress free trip! :)
Barney