Copado, hi. You have problems, you know your physical problems - they must be painful too .. but there is more I think. You say that you have been conflicted about going and are not even sure that you want to go.
I'm assuming that this has been going on for a while now? Those thoughts and the reasons why are much more important than your arthritis and top bunks as the 'top bunks problem' is probably just another invented reason not to go.
Now, it is totally normal, just before setting off, to have deep reservations .. can I do this, do I want to do this, will I fail, will they all be fitter than me and I will look stupid, where will I sleep, and the classic wail "why did I think that I wanted to do this" - these are quite normal ... we have two minds, the frontal chattering mind and the deep silent one, the observer. The chattering mind is always afraid and it demands certainty so when something really new comes up it starts to put up all sorts of excuses not to do it and produces fear - lots of fear .... the other mind, the silent observer - the 'real you' if you like, has no such qualms.
Just about every pilgrim I have known who has had those pre-Camino fears has had them just drop away within a couple of days and they are replaced with calm confidence, happiness - funnily enough, even if it is painful - and then, after a couple of weeks or so - well! the sense of empowerment, the calm courage, the knowledge that life is easy, that possessions get in the way, that one can do anything .. and then there are the friends, the supporting pilgrims, that one meets - and also the opportunity to help and support others.
So, Copado - I would say do not listen to those fears, or, listen to those fears as an adult listens to a child's fears .. be kind, but carry on - go on Camino. But don't do it as an endurance test, do it as a stroll westwards towards Santiago .. stop when tired, look at the view, carry a bocadillo and a drink and picnic along the way, look back from hilltops to see how far - how far! you have come ...... ok, say, after a week or so you find that it is not for you - so what? You will have had the experience - take the rest of the time to play the tourist in Spain - all is well.
Sorry to go on for so long here - but, I say again, the pre-Camino fears are completely normal - the actuality is so very different - and you may find that your arthritis improves as constant gentle exercise has been known to ease up and lubricate those joints. I knew an old Irishman once who cycled 15 miles a day on an ancient heavy bicycle. He had arthritis and I asked him why he cycled so far each day and he told me that if he didn't he found that he couldn't walk!
So, Copado, what do you have to lose? nothing - what do you have to gain? Everything!
Buen Camino Copado - Buen Camino!!!