Marathon Motivation: Brian Boydmanages to stop comedian Tommy Tiernan from moving just long enough to find out what possessed him to enter this year's Dublin marathon

The last time I saw Tommy Tiernan he was sitting in an infamous Edinburgh early-house pub at seven in the morning singing Syd Barrett songs with a rake of pints lined up in front of him. Now, though, he proudly describes himself as "the Haile Gebrselassie of Irish comedy" as he prepares to run the Dublin City Marathon.

"A few months ago, I started off with a fitness schedule," he says. "When I first really got into the training I had these great ideas of how quick I could run it. First, I wanted to do it in under four hours, then, a few weeks into the training, I was hoping for under three hours. With just a few weeks left, though, I realise that those were silly fantasies. The reality of it is sinking in. I'm not thinking of hours now, I'm just thinking of mustering up enough courage to finish it - in whatever time it takes."

Like many a person in the mid-30s, Tiernan was looking for a meaningful exercise regime and found it in training for the marathon.

"I just felt myself turning into a doughnut," he says. "It really is the case nowadays that we live in something akin to a zoo in that everything is handed to us. It seems like the only exercise that exists in modern life is looking for the remote control.

"I'm by no means a natural athlete, but I have tried things such as kick-boxing. Now, though, it's only running that does it for me. I live on the outskirts of Galway, so there's great country lanes to run around. I love it, I genuinely feel great afterwards."

When Tiernan staggers over the finishing line he will take a few days off, but will soon be out running around Galway again.

"You get these amazing physiological changes from running," he says. "It's almost like your body is rewarding you for doing a bit of exercise. I don't think I've become addicted to the endorphins that are released just yet and even if I have I'd only be like the first-time dope smoker to the heroin-like addiction that someone such as Sonia O'Sullivan has."

Over the last few months of pounding the streets, Tiernan says he has drawn great inspiration from others like him - unathletic types training for a very big challenge.

"I saw them today in Galway, I was driving home, it was lashing rain and there were about 40 people out running. These weren't athletic club runners - some of them were shaped like darts players - but you can only have respect for people like that."

Tiernan's marathon preparations have now entered into a different realm - he realises he's not running against a clock, but rather against himself.

"It is really odd; it is a life-changing experience in that you set yourself this massive goal which is a very, very difficult thing to achieve.

"At times, you never think you're going to do it. For me, this has now become a journey into will power. I just have to push myself through this. This is something I'm going to be so very proud of. It means I can compete and it means that whatever about any other aspect of my life, this is something that I have achieved."

What is seeing Tiernan through this last week is the fact he's told everyone he is doing it, so he cannot pull out now. More importantly, he is raising money for CD Helping Hands, a west of Ireland-based charity.

"They do magnificent work," he says. "They offer very practical support to the parents of children who have cancer and who have to travel up and down to Dublin for chemotherapy sessions. Basically, they look after your house when you're gone and look after the small, but vital things. And I'm still accepting sponsorship."