Yep, we missed our train, might have a layover at Pisa.
25 min walk or less than 10 min taxi ride.
Bummer. Remember that Google Maps will give you an answer to this kind of question instantly and definitively.
Good luck.
There's also a public bus that takes you directly from the station to the Field of Miracles where the Tower is. Google Maps will tell you which bus. Forget where/how to buy bus tickets, probably from a little store at the station.