There's too much ground to cover even for a strong cyclist willing to do nothing but ride all day.

So approach it as you would any other tour and decide where you want to go and what you want to see, then plan around that. If it were me, I'd move north and tour Florence and Tuscany, but you should plan according to your preferences.

BTW the season may be a factor, so if the trip is in mid summer, I'd definitely avoid riding farther south, and maybe even use a train to go as far north as the Plain of Venice, maybe riding from Milan to Venice or Bologna. It all depends on what you want. You really can't go wrong, except possibly a long ride along the coast in tourist season where you might see more traffic than necessary.