Not the game for you.He was playing football on a scabby rectangle of concrete in the shadow of the flats in Ballybough. Same old crack. Twenty-a-side. Last picked. Planted in goals. Dodging potshots.

Inevitable. The dreaded ball came at him, passed back, low and gentle this time and not too fast. He stuck out his foot to tame it but it scurried over his shoes like a rodent and bobbled into the territory beyond. Goal! Somewhere from under the racket of triumph and blame he heard his Da. 'Not the game for you son', he was saying. He shook his head.

Decades later, John O'Regan awoke in the middle of the night. He had made his bed the evening before. By hand. Digging a hole fit for a corpse in the Lapland snow and crawling into it to sleep under northern skies. When he awoke, his comrades were sleeping but he felt an urge to walk. He got up and wandered away from the fire towards the woods and through the woods towards a vast frozen lake. It was -30°.

He heard a call. 'Irishman! Irishman!' He was probably the only Irishman in the whole of Lapland that night, so he followed the voice. Standing by the lake was Lars Falt, head of training for the Swedish army. From somewhere under his layers of clothing Falt produced a small bottle of Jameson and two glasses.

They tapped the snow off a couple of logs and sat there in the pristine wilderness. And John O'Regan knew then that this was the game for him. He wanted more.

Then he went back to his hole in the ground and forgot when he took his boots off to put them back in the sleeping bag with him. Next morning the boots had frozen rigid.

Nature's reminder.

Can you make snowboots? Do you know where to look for berries which the birds might have left? Can you fish through a hole in the ice? Can you snare a bird? Would you know to bring candle wick into the wilderness and why? Could you distinguish which roots are good to eat and which aren't? Could you live without texting and emails and news and the constant reassuring babble of the world?

There's a man who works in Connolly Station who can. As a kid he excelled at little but knew there was a world out there which was waiting to be conquered. Ball games were beyond him. His grandad was a boxer, but apart from pummelling the odd cushion he was not encouraged that way. In Ballybough there was no grass, but his uncle loved fishing and he'd take John camping, around Aughrim in Wicklow. Fishing, hunting, the open fields, the big adventure of a long weekend.

And he took the skills back with him and fished for brawny mullet in the slimy ravine of the Tolka and never forgot that there was a game out there for him.

He was always looking for something, a flavour to make the one go round you get in this world a little more interesting. Lapland in 1998 wasn't his first survival outing, but the extremes of cold and visual deprivation and self-reliance appealed to him.

Dropped at Karuna airport and brought by bus to the end of the road - the literal end of the road, the point where there is no more road. The group of them were picked up by skidoo and driven off into the night and deposited, finally, in log cabins.

The next morning they showed them a map. It had all the usual ridges and contours and promised hills and mountains and rivers and lakes. John O'Regan looked outside and reflected that they might as well flip the map over and study the white reverse side. They went outdoors, and by childish impulse he scooped up a handful of snow to make a snowball. The blood drained from his skin. The glove couldn't go back on quickly enough. It took many painful minutes for the heat to return. No more snowballs.

He loved the feel of being in the wilderness without being a tourist.

"I'm in work, I have deadlines. I have bills. I have worries. Life is simplified out there. It's just yourself relying on yourself."

The Lapland course was run by Ray Mears, whose name you may be familiar with if you sit on your sofa every evening watching other people do extreme things. When O'Regan graduated from the Lapland wilderness with distinction, Mears pushed him out of the nest, telling him he wanted to get a postcard from O'Regan from some place exotic. Easier said than done. O'Regan went back to work.

IN CHAPTERS BOOKSHOP one day he picked one up off the bargain table. Survival of the Fittest, by Mike Stroud. By the time he put the book down again he knew he'd be sending Mears a postcard. From somewhere warm.

The Marathon des Sables is more than a mere marathon. It's not even a lifestyle choice. It's a deathstyle decision. Six days through the Sahara on your own. Hauling everything you need for survival along. It's a 150-mile race run at 120°F and 20 per cent of the terrain is sand dunes. The rest seems to be scorpions and snakes and jagged rocks.

If you're thinking of going, be wary of days four and five. Day Four is a gruelling, 50-mile stage, which many people can't finish until the cool of night. That's a pity, because it gives them less shuteye before the Bedouins take away your tent at 5 a.m. and you begin getting ready for the next day's regular, 26.2-mile marathon stage.

"Funny thing is," says John, "50 miles is just a number, so on Day Four you just keep going. When you do the marathon the next day, though, your head keeps telling you that THIS IS A MARATHON. That's much harder mentally."

You nod your head. Of course!

Not as hard perhaps as the footwear business. He made gaiters for his running shoes using an improvised design of parachute silk and velcro. Keeping sand out of your runners is important. The sweat on the soles of your feet makes the sand go hard and the hardness blisters your feet and the trapped heat in your shoes makes your feet swell.

If you're thinking of running the Marathon des Sables, here's what you do. Seek help. If you are still thinking of running the Marathon des Sables, buy a pair of running shoes two sizes too big. Insert an insole and wear thick socks for the first day or so. Than switch to thin socks. Then take the extra insole out. By Day Four your feet will be so swollen they'll fit your supersized runners snugly. A little too snugly sometimes. A race companion of O'Regan's stopped to tend his massive blisters one day in the desert and discovered when he went to get back into his footwear that his feet had swelled beyond the parameters of his runners. He had to take anti-inflammatory tablets and wait and wait.

These things - the pain and the injuries and the toil - they aren't made any better by the constant caterwauling of your basic human needs. If you need to (sorry, there is no delicate way of saying this) take a dump in the Arctic wilderness, you must squat and dig yourself a toilet in the ice first and then when it is done you must grab a handful of snow and close your eyes and try to think of those loveable Andrex puppies and ask nobody to mention snowballs.

In the desert things are different. You carry babywipes. In the evening you wipe yourself down with a babywipe and then fold it away and then next day when you are squatting and keeping an eye out for scorpions and snakes you must unfold the previous night's babywipe and put it to further use. These are things the Pampers people never show in their TV ads.

And then there is the loneliness. There is something cathartic about the self-reliance, but in the hierarchy of human needs survival is near the base. The need for social interaction and approval and even applause is never obliterated. When John O'Regan finished the Marathon des Sables he was like the other competitors: entirely alone. He loaded a competitor with bleeding feet onto a luggage trolley and pushed him towards a tented clinic which the Bedouins had made in the sand. And that was it. Nobody to share it with.

IT HELPS, OF COURSE, that John O'Regan is quite mad. He spent his stag night on the top of a mountain in Kerry. He walks to and from Connolly Station every day, which seems sensible, until he tells you that he lives in Leixlip. He trained for the Marathon des Sables by running three hours a day on a treadmill in a sauna.

When Mark Pollack, a Trinity lecturer who lost his sight seven years ago, expressed an interest in running a marathon at the North Pole last year and asked John to be his guide, the conversation with John's wife, Catherine, went like this.

- Mark Pollack has asked me to go to the North Pole with him.

- When?

- In February. I can't really say no.

- How much?

- Oh, about six or seven grand. (Actually the race cost €9,000 but John was losing his nerve.)

- Have you got that?

- I'll just put it on the mortgage.

- Listen, I don't want to talk about this.

Well, she hadn't said no. He rang Mark Pollack and said 'count me in'.

A few months later they were in Spitzbergen. As towns go Spitzbergen is a sensory deprivation chamber. Three shops that sell jumpers. Polar bears rule the area and literally have the right of way. O'Regan and Pollack waited a week in Spitzbergen, then a Russian Antonov plane landed. The North Pole shuttle flight.

He remembers they had to sign a waiver. Not just the usual stuff. More like: YOU WILL DIE TODAY, PLEASE SIGN THIS AND REMOVE US FROM ALL RESPONSIBILITY. He read the form to Mark Pollack. They signed and got on the plane.

No seatbelts. No windows. Cargo lashed down at the back. Toilet door just a flapping shutter. Oh sweet Jesus.

At the North Pole a bulldozer clears a "runway" between bags of black sacks filled with snow. The Antonov bounces to a halt. Everyone leaps off and thanks their God they are alive, and then gets hit by the cold and begins to weep. Bags and cargo are thrown off. People climb on to the plane which can never switch off its engine or it will be trapped there. Then the plane is gone and it's just some slender humans and a few tents on a floating piece of ice.

It was worse than either of them had imagined. Mark Pollack's hands froze instantly as they stood looking at the plane disgorging its cargo.

For the race, O'Regan, like everyone else, wore a balaclava with goggles over it. The race started, and like the others he began shedding layers of clothes as his body heated up. Sweat is a deadly enemy and when on the move it is best to be "comfortably cold".

After half an hour, though, sweat was coming off his face. The -39° weather turned the sweat to steam. His goggles fogged. He removed them to give them a wipe and realised that the fog had become ice. He removed a glove to scrape the ice; his hand froze. He wound up with a small letter box slit in his goggles and peered through it as he guided a blind man around the most featureless 26 miles on the planet.

How tough was it? Ranulph Fiennes only came second. The backmarker took 18 hours to finish. Which is a lot when you are in the North Pole.

Afterwards, they had a photo opportunity at the North Pole. All longitudes meet at that point and for fun they walked around the world a few times.

The next week John O'Regan was back in Connolly with the Wexford trains screeching and hooting on the tracks above and the North Pole was where Santa lived.

IN A SHORT WHILE John O'Regan will be flying off to Vancouver. And from there to Whitehorse in Alaska. He will arrive there on the 10th of February and four days later he will embark on the Yukon Arctic Ultra Race. You wouldn't think madness was infectious, but he will be part of an Irish team of four, one of whom is Ken Byrne who works in the office with him at Connolly. Ken liked the sound of what John did on weekends. He gave up smoking and drinking and started off doing a little hillwalking with O'Regan. Now this.

The Yukon Ultra is a 100-mile race from Whitehorse to Braeburn in Canada. On foot. Individuals and teams have 72 hours in which to finish. This is only the third staging of the event, and those who have done it reckon that it makes the Marathon des Sables sound like, well, a walk in the sand.

O'Regan and Byrne and their companions Fergus Hughes and Pierse Allen have been training at weekends by lugging makeshift sleds around the Curragh and sleeping out under the stars. The Yukon Ultra starts with a mere marathon through snow and ice and wind. After those 26 miles, the organisers check the competitors for signs of sanity and if they find none permit them to continue.

When it is done O'Regan will have completed marathons in Europe, Africa, North America and the Arctic. Next year he will run a marathon through jungle, and his aim is to run marathons on each of the seven continents, joining what those who have achieved this feat call the Seven Continents Club. Those who have also been to the North Pole have their own club, the North Pole Grand Slam club. He'll be eligible for that.

And so on. Seven continents. The hottest marathon on earth. The coldest. The roughest. And no part of him wishing that he'd just been good at football.

(John O'Regan and Ken Byrne will compete in the Yukon Ultra Race in aid of Downs Syndrome.)