The pain shouldn’t be too bad, especially after 3-4 days. Immediately post-op and the next day, a lot of it depends on the anesthetic technique. If you get spinal morphine and a femoral nerve block, you will probably be quite comfy.
They will give you a machine to move your knee and encourage you to get up and use the knee as soon as you tolerate, typically the day after surgery. It prevents scar tissue formation so you can actually use your new knee. I don’t think there are weight bearing precautions, as it is a new knee. It just depends on how much you tolerate. You’ll go from a walker, to crutches, to a cane.
And yes, build up your leg strength. Bike, leg presses, etc. It will help you recover faster.
As others have mentioned, there are pros and cons to having both at once. Personally I’d do one at a time. Less time under anesthesia, even if you have to “go under” a second time. I don’t think the pain will prevent you from having a second replacement, especially when you see how much better your leg feels.
Those replacements last over 30 years nowadays. I doubt you will need a redo (no offense). Good luck!