Hi, I know just how you feel. I not only have arthritis but the bone age of 80, I am 57.
As for your walking, I would do little in the morning and again in the afternoon rather than in one go and feel it is a struggle. It will only turn you off doing it and you will fail. I know this works and it can give you something to look forward to if you set yourself little targets. I give myself tea in the garden with the chooks and one biscuit as a reward (you must find your own reward system)
As for swimming, I look like a weeble. But I go really early in the morning when they do only lane swimming...... no one looks at you. See if your local pool is open early, but only once or twice max to start off with and more only when and if you want to.
I agree with the supplements of Omega 3, I also have Cod Liver oil and cook with only olive oil (as far as possible). Try and cut out as much processed food as possible and that includes margarine and any low fat yoghurts. Better to have less, than have something that has little to do with a cow. Please read labels you will be surprised at what is processed and what is not.
If it is raining and I don't want to go out, I "potter" round the house. I take things upstairs one or two at a time and you will be surprised at how much you will have actually done. The key really is don't think you have to "do" excercise, you are doing something with a little reward at the end of it. Or walk to your favourite shop or cafe. Whatever it takes to motivate you, but be gentle with your body. It will all come right if you take small steps and build from there.