Properly adjusted, a push mower takes very little effort to use. The bar should be very close to the reel, but should not touch it. It cuts by cleanly shearing the grass, not by mechanically grinding it apart.
For a while, I used Dad's old Jacobsen. I figured if it was good enough for him when he was my age, it should be good enough for me 40 years later. Unfortunately, the drive cogs wore out. I had new ones made, but the shop didn't harden them, and they wore out rather quickly.
I only had about 50'x50' of lawn to mow, and I eased through it in about 20 minutes. The thin grass would take almost no effort, whilst the thick grass would slow me down a bit.
Getting it sharpened is a pain; very few shops have the necessary tooling needed. If your wife wants the exercise, then by all means buy her the best push mower you can find. It's the *least* you can do.
"Honey? Come outside and see the new push mower I bought you. You'll get lots of exercise so you can restore that lovely figure you had when I married you. Now that we have central heat *and* air conditioning, I don't need the warmth in winter or the shade in summer any more. . ... See how easy it is to use? Oh, honey? You missed a spot back there... . "
A few more

in case you missed the redneck humor!
Fest3er