You and your knees have been through a lot in the past few years. All of that is bound to have had a debilitating effect on your general health and stamina. Try to allow yourself as much time as your body needs. You will get stronger, but it could take some time.I’m a little worried because my stamina is just not there.
On work days, do what you can, try to sit instead of standing, and do the bare minimum when you get home.