I own a gym. I deal with exactly your situation daily. As has been pointed out, motivation is key and everybody motivates themselves differently.
Perhaps the most important thing is discipline. If you don't have a lot of that, then fear is a good substitute. Fear that you won't be able to be there for your kids because you didn't take care of yourself properly !
The best thing you can do for yourself right now -even if you do nothing else- is quit smoking. That in itself is a HUGE factor. Do nothing else and you'll improve your health. Quit smoking AND start some kind of regular physical activity and you're really on your way to some noticeable improvements.
If running is going to be your activity then I'd suggest you start off easy. Get a pair of decent shoes and stop there -don't go nuts on fancy running clothes and Nike+ systems just yet. Try and run some distance 3 times a week for a month or so. Run outside if you can make yourself do it. Consider joining a gym if that's just too much for you. When you can run a kilometer without feeling like you're going to die (your time to complete is not important) then go join a running clinic at the Running Room or similar. This will teach you technique and give you like-minded people to share you runs with. There is a social/comraderie component here that is very powerful and should not be underestimated.
Most important, congratulate yourself for taking the initiative to take this on. It's not easy and you should pat yourself on the back for taking the first step, so to speak !
Reward yourself too. If you improve your running time, take some of the $ you're saving on smokes and buy something that makes you happy -reef stuff might be a good one !
If you want a goal to shoot for, I sponsor the Melissa Run in Banff and we try and take a bunch of people up for that. It's a 10K and half Marathon (go for the 10K first !!!)
You're more than welcome to come run with us. We have a good time and it's a beautiful run.
Good luck !!!