Pre-op anxiety can feel torturous at times. Cold Comfort, but once it's over, you will most likely be thinking - If I knew it was that easy I wouldn't have worried as much.
As for your questions - Naturally I was apprehensive when it came to standing the first time, but you have your walker, the prosthetic will hold you up and you normally have the coaching of a nurse or PT by your side. So...you just trust and take those first step as millions before you have.
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You will very likely make it to the bathroom on time as long as you ring for assistance and don't wait. If I recall I believe a nurse may have walked along side me the first time, but I was in the restroom and on the toilet, up and down, all by myself. You'll be surprised at what you're able to do so soon after surgery. At least I was.
Read some of the pre-op threads. There are a few others that were feeling pretty anxious and some of the responses they received may bring you comfort.
HERE and
HERE (tap/click on blue for thread links)
Come back often and share your worries or concerns and we'll do our best to alleviate your fears. Lots of comfort to you as you await Monday.
@Brookeer