Depends, if that is all you are doing then try to get more 10000 steps in as well. But if you are already, then pilates is good
Depending on your goals. If you wish to be more active i would suggest walking outside, if you want to archive a more muscular figure then yes, you should work out more. But just to make you feel better pilates is fine.
Well, that depends on if you think you can. When it comes to exercising, you need to start small and then work your way up at your own speed