I found that - in the beginning - just getting 10 minutes of walking in seemed like a huge victory and I still count it as such because those first few steps that didn't seem like much at the time made it so a few weeks later I was like: "I can do 15 minutes today" which eventually has led to strength training and 20 intense elliptical minutes 5 days a week. You can do this. You'll love yourself for the gift.
btw - also ADHD and also a former smoker - just quit 34 days ago because it got in the way of my new and improved self.