If you are super turbo fatty even mild exercise can result in injury. Definitely do both, and as soon as you can, but if you can only manage one then diet should be the focus. That is what will get you to your desired weight with or without exercise. That's all.
Walking is okay, even if you're a super whale. Can make a few kg of muscles if you're on calories deficit, high protein and walking decently. It's a good place to start, and if you have 100 pounds to wait, you don't have to wait 6 months to start. And if you just want to be healthy and live long, and have some spare time, walking only is probably fine. Walking + gardening is probably a very very good combo, if all you're looking for is a long healthy live.
Depends how strong their knees are and how super whale we are talking about. I got friends who are 6'3 and 300 pounds and still play sports... albeit kinda slowly. Other people are about the same size and struggle to get up out of a chair. The joint injuries are because you don't have enough strength to actually absorb the impacts of walking and/or running, so your joints just take a beating (plus the weight).
