Renting car in Florence, driving hill country for 2 days returning car to naples airport to catch train/bus to Sorrento. looks like longest drive will be Orvieto to Naples. does this sound doable?
If you can drive 3 hours on a freeway, it's doable. Actually from Orvieto to Naples won't even take that long. But my question is another. Why aren't you driving to Sorrento and return your car there? Most rental companies have an office in Sorrento. To me returning the car at Naples airport to take a bus to Sorrento (70 min bus ride) is crazy. It's much easier to drive all the way to Sorrento. You'll even avoid driving in Naples, which is always a plus if you are not used to driving in Italy. Although the airport has a freeway exit right there, so no city street driving is required.
Once you have a car all the way to Napoli, it makes no sense not to drive to Sorrento. As for "hill country", there is no such thing (if you ask about it, most people will be oblivious to what it means). It is like other Rick Steve's neologism that didn't catch up - the "lake district" .