Expected lifetime mileage
How many miles should I expect from a 2009 Ford F-150. Vin number 1FTPX14V59FB36821 it has 141K miles. Not sure of maintenance history
Start with an oil change, sea foam the gas tank, and replace the fuel filter. You can get 250k+ if you do regular maintenance. My father has never had a vehicle that he hasn’t put a couple hundred thousand miles on as he does regular maintenance and changes oil every 3-4K miles. His suburban has nearly 600k now.
Since your car has more than 140k on it the previous maintenance will largely determine the life of the existing drive train. Take good care of it and it can last well over 200,000 miles if the previous owner took good care of it. The 5.4 engine has a weakness in the cam phasers. If they go bad you can install a cam phaser elimination kit. In the meantime keep the oil clean which will help keep the phasers working properly.
Since you do not know the maintenance history I suggest you change all the fluids and filters. Change the serpentine belt and the radiator hoses too.