As spur-of-the-moment decisions go, it takes some beating. Jen Banfield, a 27-year-old reception teacher at Martin’s Wood school in Stevenage, Hertfordshire, resolved to run this year’s London Marathon last August, when she picked up a magazine.

“I read an article: ‘Things to do in a month: get fit, apply for the marathon’,” she says, “and that was it.” The crazy part is, she can’t stand running. Even now, eight months and many miles later, she knows “long-distance running is not my thing”. But she’s still going to do it.

“Once I’d set myself the challenge, I had to stick to it. I’d told so many people, I couldn’t back out.”

Come Sunday, April 13, she’ll be there, along with almost 35,000 runners, including more than 2,000 teachers. They’ll pound across 26 miles, 385 yards of the capital’s streets through private pain to personal glory, raising a mountain of cash for the many charities that use this mass sporting spectacle as a vital annual fund-raiser.

Ms Banfield is running for Great Ormond Street children’s hospital in London, where she was regularly treated for a hole in the heart until she was 16. As an adult she’s always been active, getting into the musical martial art ta’i bo, boxercise and polo (before teaching, she worked as a polo groom). But never running.

Now, thanks to training and advice from a professional coach, she plans to run the course in under four hours. “I wanted to aim for three-and-a-quarter hours,” she says. “But the training would have been too hard. I didn’t have time.”

Even fitting her less ambitious schedule around the school day has been “a nightmare”. Ms Banfield gets to school at 7.30am and never leaves before 5.30pm. “I have to go out running as soon as I get home, or I just don’t do it,” she says. With a long run or a race on Sundays, her life for the past six months has consisted of little but school and running.

Alex Kenny is also feeling “absolutely knackered” as the day approaches.

The 43-year-old English teacher at Stepney Green school in the London borough of Tower Hamlets is a National Union of Teachers representative for east London who spends most spare moments campaigning. With a five-month-old baby to attend to each evening, he has to squeeze training runs in at 5am. “It was hard in the winter getting up in the dark,” he says. “But if I’m going to do it, I want to do it well.”

Aaron Mellor, on the other hand, says running 40 to 45 miles a week has made him much more alert at school. “I find myself jogging down the corridor sometimes,” says the 27-year-old, a design and technology teacher at the Minster school in Southwell, Nottinghamshire. “And I have better relationships with the kids because they want to talk to me about it.”

Nevertheless, Mr Mellor, who hopes to raise pound;1,500 for the Asthma Campaign, says the all-consuming nature of the training means this will be his “once-only” attempt. “You get obsessed with it,” he says. “All I think about is the food I have to eat and my next run. It’s taken over my life.”

A year ago, Joe Kwaterski would also have sworn that once was enough.

Headteacher at the Orchard school in Shropshire, a small, independent residential school for children with severe learning difficulties, he finished the New York marathon in 2001 and vowed he’d never do another one.

Apart from the desire to raise more money for Mencap (he raised pound;3,500 last time), he’s still not sure why he has relented. “I think I’ve lost my senses,” he laughs. “But then, I suppose we need to set ourselves little targets to prove we’re still living. We find ways of putting meaning into life, and this is my reassurance that I’m still here.”

Mr Kwaterski, 53, caught the running bug at 18 because he wanted to do something active that wasn’t competitive. It’s his therapy, he says. “It makes me a better person. Running helps me resolve all the problems I’ve had in the day. If you spend all your time on cerebral stuff you need to balance it. Running is the closest I’ll come to meditating; it keeps me human.” His first marathon took place less than two months after the September 11 attacks. “You could still smell the ash in the air,” he says.

“Everyone was so pleased we were there. It was phenomenal.”

London will have to be pretty special to match that, he says. But then, for him and thousands of others, just crossing the line in the Mall, and still feeling human, will be special enough.

www.london-marathon.co.uk

MARATHON FACTS

Teachers taking part

* 1,849 teachers ran in 2001 (706 women, 1,143 men)

* 2,064 ran in 2002 (854 women, 1,210 men)

* 2,035 are running this year (897 women, 1,138 men)

* There are more people from education running this year than from any other occupational group, except students Charity begins on the streets

* pound;31 million was raised for charity in 2002 by 76 per cent of the runners

* An estimated pound;187 million has been raised for charity by runners since it started in 1981 Boom town

* The first London Marathon was held on March 29, 1981, and 6,255 people finished

* In 2002, 32,899 runners finished

* More than 500,000 people have completed the race since 1981