bob,
The quickest and most efficient method would probably be to take a budget flight from Pisa to Paris. EasyJet operates on that route and currently has flights advertised for about €52.49 PP.
You'd first have to take a train from whichever of the Cinque Terre towns you're staying in to Pisa, but that's a relatively easy trip. Upon arrival at Paris / ORY, you would travel to Gare St. Lazare to connect with the trains to Brittany. The easiest method is probably the OrlyVal People Mover to Antony station, then RER "B" to one of the stations in Paris, and then connection to RER "E" for the short trip to St. Lazare.
This trip is still going to take the better part of a day, but it will be more efficient than a 15-25 hour train journey! There may also be flights from Pisa to Dinard using RyanAir (which I'm always reluctant to suggest) with a stop over in London, but I didn't check the details.