I too smoked when I first started running. I smoked through my training for my first and second half marathon. Two days before my second half, I had one cigarette in the pack and decided to smoke it, but not buy any more until after the race. I've always been a slow runner, but my time improved by so much and I felt so good, I decided right then and there to quit. In all honesty, I did have a slip-up a couple of weeks later, but thanks to a nasty sore throat, I stopped smoking for the few days it took for my throat to feel better. When I lit my next cigarette, it was so stale and nasty I gagged. That did it for me and I haven't smoked since. I still have the rest of that pack in my car in case I ever have the urge to smoke again. After three years, I'm sure they're real tasty now!
From friends and family that have quit or tried quitting, most everyone had agreed that quitting cold turkey has been the best for them. My sister-in-law said when she tried the step down method, she went through withdrawals each step down. Quitting all at once meant withdrawals once and then she was fine.
If you can try not smoking for a couple of days and then go for a run. You'll be amazed at how much better you feel. That should help you get through it after that. If not, post. There is a lot of support in this community and I'm sure everyone would be willing to show you the support you need.
Oh, be prepared for the clearing up of some of your senses. Taste and smell being the big two. The smell of cigarette smoke now is horrible and I can smell it on anyone who smokes.