My hardest ride to date was the 82 miles of Western Express from Vallejo Ca to Rancho Cordova. I was not carrying anything and the climb out of Vallejo was very hard for me.
I just generated a RWGPS route that looked like 10 miles of climbs (some steep, nothing too high) followed by a lot of flat riding. There's a saying in the long distance cycling community that what you don't have in your legs, you need in your gears. If your bike doesn't have gears down to about 20 gear inches, that's one good place to start.
I've written, only half joking, that the easiest way to ride the TransAm would be to fly into Wichita, ride to the Pacific, fly back to Wichita, and ride to Virginia. That would give you the chance to build up on flat land before you hit the Rockies, and after that you're ready to face the eastern mountains.
For shorter rides, you might consider tackling the Mickelson Trail in South Dakota. Another week or two ride would be the Natchez Trace, ride up to Nashville (getting into shape before you face the last 100 miles of hills) and perhaps head back south.
There are also some supported tours around Michigan, Wisconsin, and Minnesota you might want to check out. There's some hills, yes, but they're not that long, and somebody's hauling your luggage for you. It's a nice way to enjoy some cooler cycling in the middle of a long, hot summer.