Even in a sporting discipline, which thrives on the spectacular, it has come to be acknowledged as one of the more extraordinary stories of the decade.

As a young man, Marcus O'Sullivan was too small to survive in field sports - not fast enough to win a schools' race at Colaiste Chriost Ri in Cork. Yet over the next 20 years he would push himself again and again through the pain barrier at speeds which another generation considered inhuman.

And this evening this improbable athlete stands at the portals of one of the most exclusive clubs in sport, a club so elitist that it numbers just two members. Only John Walker of New Zealand, the Olympic 1,500 metres champion at Montreal in 1976, and America's Steve Scott have gone where he now aspires to go.

New York's Madison Square Garden is the setting and the Wannamaker Mile the event as O'Sullivan steels himself for the last great challenge in an exceptional career and seeks to run his 100th sub-four minute mile.

"It could be a great day for me and hopefully, I'm ready for what it may bring," he says. "For some time now, it has been apparent that I had a chance of joining the ton-up club. And if it happens, it couldn't be in a better meeting or in front of a more appreciative crowd."

It's easy now to forget that less than 50 years ago the four-minute mile was being linked with Everest as targets at the very edge of human capability. Roger Bannister's singular bravery at Oxford in 1954 would puncture the myth of impossibility, however, and open the gates.

To achieve it even once is still a gratifying experience for the vast majority of athletics. To do it a 100 times, even in this era of spectacular conquests, is to approach the realms of disbelief. O'Sullivan, never boastful and frequently selfdeprecating, refuses to indulge himself at the prospect.

"There is and probably always has been a group of sublime performers who over a two or three year period are capable of anything," he says. "It was never in my destiny to be that kind of athlete and I'm not at all sure I regret it. When you've been to the pinnacle it's very hard to go back there, and over the years I've seen many great careers fall apart.

"My strength is in my longevity and the fact that I've been fortunate enough to be able to go on grinding out reasonably good runs. You weigh those two options in the balance and you take your pick.

"I never had the physical attributes to be a great athlete - certainly not to the same degree as Niall Bruton. But by developing the mental strength to go on doing the same thing, and trying to do it better with each passing year, I was fortunate enough to enjoy a good career.

"It has taken me almost 15 years to put together my 99 sub-four minute races. And from the very first one at Chapel Hill in North Carolina in 1983 I have felt enhanced by the experience."

The high points of his career are identified as three consecutive world indoor 1,500m triumphs; notably the one in Budapest in 1989 when a huge contingent of Irish football fans, who had travelled for Ireland's World Cup meeting with Hungary, turned up to produce a whole new atmosphere for an indoor athletics meeting.

There was, too, the immense satisfaction of joining with Eamonn Coghlan, Ray Flynn and Frank O'Mara to set a world record for the 4xmile relay at Belfield in the final testament to a remarkable Irish tradition in the showpiece event of the track.

Woven into a fine international career are the memories of some great nights in Madison Square Garden where before last year's mini-disaster his record in the Wannamaker Mile stood at five wins and six seconds. For some inexplicable reason it all went wrong for him last year when for the first time in his career he finished out of the first three in the race.

It is against that background that he takes his place on the starting line this evening. History may beckon but only it seems if the traffic doesn't get too congested on this notoriously tight four lane track.

As of yesterday there were seven declared runners, including another Irishman, Shane Healy. But the speculation is that it will be increased today with the inclusion of the brilliant Kenyan Leban Rotich.

"It's shaping up like a fast race," says O'Sullivan. "I just hope that I get a clear run and that at 36 the old legs are still up to it over the last 200 yards."