As long as you don't smoke during the race I suppose it's no problem, each to his own
I can remember turning out in the 80s to support my dad running the Glasgow marathon and seeing dozens of blokes puffing away at about the 15 mile mark without breaking stride. A lot of them were in front of him and he was running 3:15 - 3:30.
giving up smoking is easy
Yeah, I've done it dozens of times. LOL! 
Do agree that telling everyone it's so hard to give up makes it very easy for folk to fail and not feel too bad about it. I've always suspected the 'harder to quit than heroin' thing was started by the big tobacco companies. Each time I stopped I just went cold turkey and didn't think about it after the first three days or so. Usually went back to it by drunkenly accepting an offered cigarette without thinking. The ban on smoking in pubs was probably what made me chuck it for good a few years ago...