Russel, It's going to be a very nice building, although you may have to build another one yourself. I doubt it if your wife will ever give it up.
The way I square a building is to lay out all 4 sides equally, and use a string level or builders level to find the point where all 4 posts are level. Then measure it again. If you don't measure every point on the same plane, the lengths will be different. I then measure diagonally, and when the 2 diagonal distances are equal. the building will be square and level. I always stuck the poles in the ground, making sure I had a little more than I needed above ground for the walls. I would then cut them to the proper height after the walls were plumb and square, before putting the tin on the roof.
I know what you mean about the joints aching, the back and feet hurting, etc. At one time in my life I could work all day long and only darkness would stop me. It hasn't been like that for a while.
Years ago I wore my beard a little longer than I do now and I could use my beard as a pencil holder. It was fine until I would forget about the pencil and go to a restaurant and eat. I got some strange looks!
When I sometimes went a little unkept, I could place 2-3 pencils and use them as spares. Ah, those were my wild and wooley days.