By Fred Brenton BBC News website reporter

The BBC's Fred Brenton hopes to shape up and help charity

Just over a year ago I turned 30 and realised the man and boy who used to resemble a slightly heavier version of Rodney Trotter had been replaced in the mirror by the Pilsbury Dough boy.

In fact, on more than one occasion, old friends reminded me of my resemblance to the marsh-mallowed monster from Ghostbusters. At at that point enough became enough.

Out came the weighing scales and to my surprise I realised I actually had become the Giant Haystacks of my peer group.

The thought of dieting crossed my mind but nibbling on rabbit food and cutting out pints of the good stuff to keep the beer-belly at bay would only be short-term solutions.

I figured out that shaping up was the only way forward to beat the onset of the middle-age bulge, which I'm reliably informed takes hold at about 35.

You only have to run one once a year and people think you're fitter than Lance Armstrong Fred Brenton

After mulling over other ideas for the next few months, a friend mentioned he was running the Cardiff Marathon on 3 October and it all fell into place.

What better way to stop myself turning into Jabba the Hut before I reached 40?

There's more fat on an oven-ready chip than you can spot on top professionals like Paula Radcliffe, and Jimmy Saville's hair might be a bit of a mess, but he looks pretty good on running 26 miles every now and again.

So the plan was to sort of split the difference between Paula's ultra-professionalism and Jimmy's ageing frame and somewhere in between I'd surely be able to keep everything on an even keel and not have to turn to elasticated trousers in the next few years.

On top of that, you only have to run one once a year and people think you're fitter than Lance Armstrong and are well impressed.

But, as far as I'm concerned, if you're going to do something for yourself which will raise people's interest and potentially open their pockets, it might as well help a few others along the way.

My own chosen cause is the Cystic Fibrosis Trust (CF Trust). More than a few years ago my awareness of this life-threatening disease came into sharp focus when two of my nieces were brought into this world with its suspected symptoms.

The marathon passes some of Cardiff's landmarks

Despite initial fears they had inherited this killer genetic disease, which mainly affects the lungs and pancreas and clogs them with thick, sticky mucus, they were fortunately found to have been born with a much lesser form of it.

However, the idea that a family could be deprived of any young life by a condition which currently has no cure has never really left my mind.

But for a chance happening of biology, two of my youngest and closest relatives could have joined the 7,500-plus babies, children and young adults across the UK who currently have cystic fibrosis (CF).

The fact that they haven't is neither here nor there because there are still those thousands out there who have to live with CF on a daily basis.

One of the main objectives of the CF Trust is to fund and find a cure for the disease, which leaves people who have it with an average life expectancy of 31 years - the exact age I am now.