Why not just continue by train?
There's one at 6:15 which would be a push to get to and one at 7:15.
Trains to Lucca leave at 6:10 and 6:38. The latter would be more reasonable if you didn't already have a ticket. If you bought your ticket from wherever you start through to Lucca you might have a chance of making the 6:10 connection.
We thought bus might be easier and get us closer to our hotel, which is within the walls. Can you buy tickets ahead of time? We might do that; just to be safe. I'm not opposed to a private car either; any thoughts there?
You need to check the Lucca map, just because the bus station is within the walls doesn't automatically make it closer to all accommodation.
The bus makes two stops in Lucca: at the train station and at the bus depot, which is just outside the walls on the western edge of the old town (Piazzale Verdi). Most old town hotels are within walking distance of one of these stops.
If the bus schedules are not convenient, just take the Trenitalia unreserved regionale train to Lucca.
If your train arrives in Florence on time, you'll make the 6:15 bus to Lucca. You cannot purchase tickets in advance. Buy them at the ticket office in front of the bus stop in Piazza Adua (adjacent to the SMN train station, exiting on the side of McDonalds's and the Pharmacy).
When you exit the SMN train station (on the side of McDonald's and the Pharmacy), look to your LEFT and you'll see a number of blue buses parked perpendicular to the street curb. That is Piazza Adua, where Lazzi has a ticket office and terminus of its bus routes out of Florence.
It takes about two minutes to walk from the SMN train station (Pharmacy) to Piazza Adua bus depot.Edited: 28 April 2012, 17:11
Thank you - very helpful and were the answers we were looking for.
Further to GAC's comments. The bus stop at the Lucca train station is OUTSIDE THE WALLS. The bus stop at Piazzale Verdi is INSIDE THE WALLS, though just. Unless you are burdened with a great deal of luggage, the walk from either is easy, but there is validity to checking the map to locate your hotel and it's proximity to train station or bus station. There are almost always taxis at the train station and it'd be a reasonable ride from the train station to your hotel, if you'd need that option. There are taxi stands at the bus station, but frequently there are no taxis.
That's right, the bus depot in Piazzale Verdi is just inside the walls of Lucca on the western edge of town! My memory was faulty, and I was off base by about 100 meters!
Depending on where your hotel is, the train station can be just as close as the bus depot even though outside the walls. I can walk from the train station to P. San Michele in 15 - 20 minutes and I'm not a fast walker and am 50+. If you have heavy luggage then it is easier to get a taxi from the train station than the bus depot and not very expensive to get to central Lucca by taxi.