Hello there - and welcome to BoneSmart! You don't share much about yourself so it's not really possible for us to address your concerns directly. Is it the anaesthetic/surgery you're worried about or the rehab after? Or both!
Many, many people are out of condition for the simple reason that, after many years of disability due to their arthritis, they haven't even been able to walk much let alone exercise. But doctors are well used to working on people like this. It's not going to be much of an issue so please don't worry about it. And as many are in their 'autumn years' they often have other ailments like heart disease, diabetes, you name it! These are also well within the doctors' abilities to handle. It's what they spent years training for.
As for your overall fitness, well, at this moment you are what you are and like many before you, you have to start your rehab from where you are at. But to be honest, I doubt you'll fail to make good. It's amazing how a body can bounce back when the origin of the pain and disability is removed.
We'll be thinking of you on Tuesday and sending up prayers for you. In the meantime, feel free to bombard us with questions - major, minor, trivial - we takes'm all!