Its not the brake booster. Pedal is only hard because the brake has been pressed when the truck wasn't running. To start the truck the pedal only needs to be pushed a very small amount, enough for the taillights to come on.
My starter recently went out with only 60k miles. Brake pedal was stiff after pressing it a couple times trying to start it when it wouldn't start. It would start after 20-30 tries of starting it, so it wasn't completely dead but on its way out.
Check the basics first (starter, engine battery, key fob battery, pedal sensor, etc). First thing the tech should do is give the starter a good whack with a hammer and see if truck starts.
Basic diagnoses now days has gone to the wayside