Extreme obesity

I think someone would need a crystal ball to tell you about immobility. I've seen some younger people well over 600 at average height who could get along quite well. I've seen some people struggle at 300 pounds. Remember, that as you get older, the less likely you will be mobile. If you are injured, you may lose your mobility. So, it is not only weight, but age, lack of injuries and just plain old luck to stay mobile. At your height, you should be OK at 500 pounds. Slow, but mobile.

Most of the people I've seen deal with really high weights well are people who were fat for a long time, giving their body time to adapt.....sounds like you have that one covered?

But the heavier you get, the less margin you have-- ie the easier it is to get hurt. One mis step can lead to a wrenched knee or sprained ankle that is a lot harder to deal with if you are very large, which in turn can lead to a rapid loss in mobility (talked to more than one person who went through this). Also knees, ankles, feet will get sore more and more quickly. Wear really supportive shoes, do stretching and varied excercises, and you can keep that aspect a bit lower, but physics and biology can only be held off so much.

Good luck, but please proceed with caution!

Yeah I think exercise now would be a great benefit if you want to get 500+.
I would build up core strength and legs. As when you think about it. 500lbs is alot of sweight to be supported. I hear swimming is good. I would talk to some of the bbw modles who are of that size.

Why don't you try walking around with a 50-lb backpack and see how it feels? Or a 100-lb backpack? Or more?

Obviously, it will be harder than actually being that much heavier weight would be, since if you gained the weight it wouldn't be as sudden a change and your body would have time to adapt. But it might give you some insight.