You are where i was 2 years ago! I smoked and i couldn't do 1 sit up! I joined the gym, eventually quit smoking (1 year this past Christmas!) You WILL get to the point of being able to do military sit ups before you know it-even if you DO smoke! :-)
The hardest part is taking the first step, by joing a club you are comfortable with and congrats on that!
Keep up the good work.