Hi I'm new here too and have trouble with feet so I understand the need to find something different. I think, like someone else said that most of the battle is in our minds. Knee and feet problems are completely real but not allowing yourself to use that as a reason to not lose weight is the key.
I used to love to run and walk too and I'm also trying biking, like another said, but so far I don't love the bike enough to work hard that way. Maybe it will work out better for you. One of the things I'm doing that won't hurt my feet or your knees is doing a whole lot of core work and upper body resistance. If you move from one exercise to another quickly you can get your heart rate up, plus you build more muscle which will help your metabolism.
I lost 50 pounds 2 years ago, but should have lost 70. Since then I've put 20 back on and so now I'm here to take off 20 and then finish the job by taking off that last hard 20. Good luck to you.