Struggling, you are on the right track and I applaud you.

You are probably getting in 30 minutes without realizing it. You exercise, after a fashion, anyway, even when you aren't specifically "working out." Someone already mentioned things like parking farther away from the store, and so on. That counts. If you can walk around the mall, that counts too.

Others have already said, go at your own pace. Only you know what that is. Challenge yourself, sure, but don't do anything that will cause you injury.

Peg mentioned yoga, and I think that's worth trying. There are lots and lots of good beginner resources out there. Also, depending on your disability, Pilates might be helpful. Beginning Pilates is not at all strenuous, but is very good for strengthening core muscles. It's similar to yoga, but has more of an emphasis on strength training.

Also, can you dance? Dancing is good exercise that doesn't seem like exercise.

Good luck!