Bike trails

All the states the OP mentioned are hot and humid in summer. Why not come to the cool, colorful Colorado Rockies for the summer? Hundreds of miles of old narrow-gauge railroad grades to bike, some climbing over the Continental Divide, but others with fairly gentle grades. Plus hundreds of miles of designated bike paths going just about anywhere you want to go. Colorado is, after all, the original site of the Red Zinger, Coors Classic, and US Pro Challenge bike races and the home of Olympic and professional bike racers like Davis and Taylor Phinney and Connie Carpenter!

Amazon sells a book of route maps called Colorado Biking Trails that will get you started. And if you need some driving route help plus dispersed- boondocking, campground, and sightseeing ideas, see my blog HERE.

Though not mentioned in your list of states but mentioned by soren is South Dakota. We enjoyed the Mickelson Trail that run for 109 miles through the Black Hills.

https://www.traillink.com/trail/george-s-mickelson-trail/

A bike trail we enjoyed in Alabama is the Chief Ladiga Trail with continues east as the Silver Comet Trail into Georgia. We rode the Alabama section which was once a railway converted to bike/hiking trail and so is fairly level. About 61 miles total of both trails.

https://www.trails.com/us/al/talladega-national-forest/chief-ladiga-trail

LBL in Kentucky has a paved trail in the north end and a packed gravel trail more in the center that runs about 11 miles. Both have some hills, but nothing crazy.

X2 on Dick B's comments. We spend our winters near the Withlachochee trail. It's a great trail. That said, I can't imagine being anywhere near the area, in mid-summer. 100 degrees and/or 100% humidity isn't something I would do intentionally, which is why we ride the northern half of the states, once spring rolls around.

Last year we did a month of Indiana, Illinois, Iowa and South Dakota. We stuck to rail trails, spent a few days in each location, and had a great time. Plenty of ways to do the research and reviews online. We are members of the railstotrails.org which has great info. for finding trails to suit your needs. Have a great summer.

I assume you are riding the trail along US1 in the keys. As posted above, I also have a copy of Florida's paved bike trails.... has lots of good ideas and places to explore.
We go to Maine pretty much every summer and ride the carriage roads in Acadia National Park. While they really aren't level, they were built for horse and buggies, so the grades aren't really bad, although , coming from florida, they seemed like it at first. I think there is about 40 miles of carriage roads in the park.

There is a book called Florida's Paved Bike Trails. We have a couple of favorites: Withlacoochee State Trail and the Pinellas Trail. We stay in nearby campgrounds and ride almost the entire length of the trails over a few days. We also know where the good restaurants are along the trails.

Level is nice but you won’t improve without hills. Learn to spin.

I use Traillink to find bike trails. It sounds like you want rail trails for nearly level pedaling.