Chewie, I ran my first 2 Great North Runs (half marathon) while still smoking, just stopped for a few weeks beforehand. Then stupidly 'rewarded' myself with a smoke afterwards.

I ran these in 1 hr 50 mins, the 3rd year I had been stopped smoking completely for nearly a year, I knocked 20 mins off my time for the next 7 years and even got under 1 1/2 hours twice! (and in my forties!) Now that's serious running!
Once you get into a routine it becomes easier, the hardest part is getting out of the door and getting that first mile under your belt. In the begining it's more important to get out running regular than how far you run, obviously with a marathon as the target this will increase on a proper training program. That's why it can be beneficial to have a running partner who can drag you out when you don't feel like it, and likewise you can drag them out too.
I still run a few of times a week anywhere from 5 to 10 miles and still thoroughly enjoy it (and helps keep the weight in check too).
Warning: It can become addictive! Oh, and use decent quality trainers, the rest doesn't matter too much but decent trainers are important.
My best wishes & good luck to you.
