Florida is not a particularly large State by US standards and is approximately 350 miles (560 km) across at its widest point but it is still nearly 800 miles (1,280 km) from Pensacola to Key West.
North South Highways
The main north/south highways into Florida from Georgia to the north are Interstate 95 which runs down the east coast passing through Jacksonville and on to Miami and Interstate 75 which runs south through the central region across to Tampa on the west coast and then back across to the east coast at Miami.
If you are travelling from the North Eastern seaboard, then an easier option is to take the Amtrak Autotrain all the way to Sanford just outside of Orlando.
East West Highways
From Alabama to the west, Interstate 10 runs west/east from Pensacola on the west coast across to Jacksonville on the east coast. A second interstate, Interstate 4 bisects the State from Tampa on the west coast to Daytona Beach on the east coast, passing through Orlando en route.
Turnpikes (Toll Roads)
There are also a number of toll roads (turnpikes) such as the Beeline Expressway from Orlando to Cape Canaveral and the Osceola Parkway which provides quick access to Disney World.
Remember to keep a handful of quarters in the car to pay for the tolls and bear in mind that Florida's toll roads are gradually moving over to electronic collection; ie. no cash tolls.