I'm no big century rider, completed a couple. But for comparison, I rode one here in western NC which is mountainous, and two months later one on the coast which is flat, I suspect rather similar to what you'd experience in Florida. Without getting into specifics I believe the flat ride is no easier than the other. While much energy is spent in climbing, you can at least reap the benefits of the elevation gain by resting during the downhill coasting. On the flat terrain, there is constant pedaling & little chance to coast any length of time for rest.

If you completed the "horrible hundred" a couple weeks ago without issue, I doubt you'd have much trouble with a few more thousand feet of climbing.

I realize none of this answers your training question.