We just did the Louisville KY 600K and it had over 25,000 feet of climbing. We live in flat northwest Ohio, so it was super challenging (even compared to the Ohio brevet series that had hills). Only argument that convinced me to finish was "if we don't do the final 200k now, we'll have to do 600k later."

We rode the entire Natchez Trace two years ago over Christmas holiday. The northern section does have some hills - we found them enjoyable and not overly difficult even on our tandem.

Advantages of the Natchez Trace - very little navigation required (may also be a negative since I find searching for the next turn to keep my attention). Little traffic, few encounters with dogs, pretty if somewhat monotonous scenery. There are restrooms with water and a couple nice campgrounds. I'd feel pretty good about riding in the dark there. And out and backs let you occasionally see a fellow randonneur.

Disadvantages of the Trace - absolutely no services along the road itself. You will encounter dogs on roads to check points.

Let us know what you decide to do. We are thinking of doing the 1,000k which starts on Thursday the day before the 600k.