I'm not an expert... but I think walking would be perfect for you. Doing anything that hurts you can do some real damage, plus you're not going to want to keep it up if it hurts! For me, I'm walking now, but might try running when I get down below 200
pounds (so it's not so hard on my joints). Let your
body tell you what you can do. You want to push a bit, but not hurt yourself.