It turns out that some airports are bigger than others but it is pretty easy to get 1500 Fitbit steps in just walking from the parking lot, through ticketing, customs, security (do Fitbits set off the metal detector?) and onward to your departure gate in a smaller airport. The same after landing. In bigger airports there can be much more walking and the step count can get over 5000. Stay away from the people movers wherever possible and travel safe!
If you want to get 10,000 steps in on a travel day you're going to have to plan an extra walk in your day either before or after your flight!