I would say prepare to be very tired! An hour long private lesson for someone who has not ridden much lately will be very tiring. Let your instructor know of you physical shape and that you may need to ask for a breather from time to time. It's easy for the instructor to get wrapped up in calling out instructions to you and miss seein how red in the face you are and how you're sucking air. It is demanding, and you need to take breathers.
If you can do some walking, where you walk fast and up and down hills, and some core muscle excersizes, this will help.
Smile a lot! And when you are done say, "That was a GREAT lesson!"