I got fit while smoking. Start walking. Like Fabian said go faster and longer each time. When I first started I set a 100mile goal. I started 1 mile at a time and it took 3 months. By time I met that I was in so much better shape and could do much more. I walk 100 miles in 2 weeks now.
I smoked for years and I actually used fitness as a distraction during my quitting process (quit 14 years ago and never once relapsed). Anytime I was craving I would get up and move, walk, workout video, anything, because I didn’t associate smoking with working out. Maybe you can do both. Definitely start with walking and just do what you can.