I'm going to give you a short version, then I'll update it with a longer version later this evening, since I'm about to head off into bubble bath land for a good hour or so...
Leaving Venice, the only way out is either by boat or train. You can leave out early morning for sure with your bike by train. Not every train will accomodate your bike, but I believe the early morning train does.
Take your bike to Verona, and hang there for a day if you can. It's one of my favorite places in Italy. Verona has it's own highlights that are spectacular. From there, you can ride westbound on the slower highways that allow bikes along the highway that goes past Lago de Guarda (I will have to look that name up just to be for sure) headed into Brescia (another spelling to look up). That's about 70 km from Verona. If you want to make a big push, you can ride an additional 70 or so km to Milano. It's almost all flat, so if you're a strong rider and you aren't carrying a lot, it's totally do-able.
From Milano, you can stay overnight (but it's expensive), then pick your way through the city and head southbound towards Florence. It's here that you'll hit hills, but let me go into my storage where I keep my maps tonight and look at some less hilly alternatives for you. From Florence, it's not a bad hop into Pisa. I cannot remember for sure how hilly it was, but again, I'll check my maps and see what you can do.
Forget about a bike path. Ride the streets. It's the only way to go. There just aren't any real bike paths (like what you saw with that Holland post a few days ago). They sometimes have paths on the street or along the sidewalks, but they are rarely used, except by pedestrians who are jogging. It seemed like everyone who had a bike rode the streets. I never had a problem during the six and a half weeks I toured Italy. If your legs are feeling like deadwood after riding for some time, you can always chuck your bike on the train and ride it for however long you want.
Short answer. I'll look up some information tonight and PM you with some alternatives.