It's a really hard thing to do when you struggle to walk. Do you have inhalers? If you do, do you use it before you try to walk? Don't go mad to start, just go for a couple of minutes if thats all you can manage. If you can only do 10 steps do that for a couple of days then make it 11 steps for a couple of days and carry on like that. I can do more than that but to start with it was really hard. You have to challenge yourself but don't push yourself too far. I found it helps with the depression bit too as you get a sense of achievement for every extra step you do. You will find it easier if you manage to lose even a pound too again you have achieved something really good.
Don't think of it like a massive thing, just take tiny steps (not literally), don't say you need to lose a stone, just a pound, don't say you need to walk a mile just one extra step. You wouldn't make plans for the next 10 years just the next few days so use the same idea with exercise. It is so important for us to exercise but just don't set yourself up to fail, set yourself achievable targets and let us know how you get on.
Good luck love, remember, one step at a time xxxx