What a superb idea! I think you should go for it! However, two remarks spring to my mind. First, I agree with andym that cycling through Italy might not be the easiest or best route.
That wasn't what I meant to say... Italy is superb (and I loved the Gargano too). If I were heading through Italy North to South (and time wasn't too pressing) I would sort of follow the chain of national and regional parks that run along the Appenines - eg Monte Sibillini , Gran Sasso, Majella, Gargano, Alta Murgia etc going down through Umbria, Le Marche, Le Abruzze and into Puglia (and on then to Greece via the ferries from either Bari or Brindisi). Not the most straightforward route planning wise, and definitely not the flattest but probably the most scenic.