Thubron, head honcho at TBWA\Hong Kong, will be running over 40km alongside, or possibly slightly behind, the likes of Olympic gold medallist Henry Wanyoike, having pledged to take on the feat when topped up to the gills with Dutch courage that fateful evening many months ago.
Having successfully completed a half marathon in Shanghai in November, the whole shebang appeared to be the next logical step. Or it did at the time, Thubron ruefully remarks.
Hong Kong's society watchers should take note that Thubron, who has gained a reputation as someone who likes to dress up, will on this occasion be sticking to regulation wear. "It's hard enough to get round the course in anything more than shorts and a T-shirt, let alone anything ridiculous," Thubron shudders.
Could this mean the party circuit regular is hanging up the dress for good? "Transvestite Ian remains in the 1990s," the athlete proclaims.
"I am now a married man with three children. That was the young wild Ian. He's gone for good. He couldn't have lasted a bloody kilometre, let alone a marathon."
Having embarked on a disciplined training schedule a trimmer Thubron is ready to live up to his vow, standing to raise HK$100,000 for WWF in the process. Provided this Sunday doesn't prove too traumatic the new healthy lifestyle will remain. Follow-up marathons however are not part of the plan. "This is definitely a one shot wonder," Thubron assures Dairy, wary of what happened the last time he made an impetuous promise. "After this it will be nothing more demanding than afternoon jogs on Bowen Road."