IT IS a measure of the exalted status of Mo Farah that he is being talked up as a potential British and even world record-breaker over a distance at which he is untried at tomorrow's Virgin Money London Marathon.

Never has such high expectation ridden on a British athlete's debut.

The double Olympic and World champion at 5000 and 10,000 metres is the UK record holder at 1500m, 5k, 10k, and half marathon. He has run faster than Coe, Cram, and Ovett (all world record holders) at 1500m, and has surpassed the former world record-older Dave Moorcroft at 5k. This prodigious range has perhaps made us blasé. We risk suspending logic. For me, the jury will emphatically remain out until Farah crosses the line in The Mall tomorrow.

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The fastest marathon time by a UK runner (2:07.13, in Chicago) was set by Steve Jones of Wales in 1985. The fact this record has survived for 39 years speaks for itself and Jones, the last Brit to hold the men's world best, now ranks only 158th equal on the world all-time list.

The world record is held by the Kenyan Wilson Kipsang at 2:03.23 (Berlin last year), though the fastest is 2:03.02, achieved in Boston on a point-to-point course which drops 139 metres and is consequently ineligible for records. Jones says he won't be surprised if his record falls. Nor will I, but victory will be a stretch in the strongest field ever assembled. Eight men in the line-up have run under 2:05 and the marathon mark has improved by 2min 15sec (six times by five athletes) in the past 12 years.

Farah has the greatest endurance runner of all time as pacemaker tomorrow, Haile Gebrselassie, and his presence is a reminder that Farah should not look for too much, too soon. A multiple world and Olympic champion at 10k, and twice a world record-breaker at the marathon, Haile was also holder of the world best over 13 miles. Yet he had to wait for his seventh marathon (including a London drop-out) before breaking the world record.

It is a mistake to set excessive store by half-marathon performances when assessing potential at the full distance. Zersenay Tadese has won five world half-marathon titles and holds the world best at 58:23, but has yet to beat 2:10.00 for the marathon and does not rank among the top 500 all-time. His predecessor as world record-holder for the half, Sammy Wanjiru, is not in the top 20 at the marathon, and the UK record-holder at 10k on the road, Nick Rose (less than a minute behind Farah at the half), never broke 2:21.

Farah, however, will be encouraged by the result of the Paris Marathon last weekend. His old Ethiopian rival Kenenisa Bekele won on his debut at the distance in a course-record 2:05.02, outstripping previous bests held by Haile, the former world record-holder Paul Tergat, and the 2008 Olympic champion Wanjiru.

Jos Hermens, the delightful Dutchman who manages both Haile and Bekele, said he had, "seldom seen anyone recover so quickly, jumping up and down stairs," after Paris, but he cautioned against world-record hype. "You can do as many long runs as you like, but you can't train for that area after 35k," he said. "That's unknown territory until you race it."

When winning in Paris, he said Bekele "ran like a track racer. He put in a couple of fast kilometres - he should not have done that. But you have to learn. He could have run faster, maybe 2:04.30. It was a great debut. But you can't compare it with guys who have run marathons from the beginning, guys who never trained for the track.

"Track runners take time to adjust, because they have years of speed training in their legs, and the whole system has to change. I think Mo will have that same experience. Anything under 2:07 would be great."

The marathon duel between Farah and Bekele is one London guru Dave Bedford anticipated years ago. "I hope it happens," said Hermens. "Kenenisa is ready for it, sooner or later."

Anticipating Haile (41 this week) in London fills me with that sense of foreboding which attends watching a great heavyweight champion going back into the ring. In his pomp, he was peerless, with a range to match Mo. He was world indoor 1500m champion 15 years ago.

How much longer can he continue? "He just loves it," says Hermens. "The guy is addicted. He will run his whole life. I can't stop him. I've tried . . . the arguments that he is such a big name and should not do this pace the race. But he pleaded: 'I've set 27 world records. Many athletes helped pace me. Why would I not do this for them? London was never my best, because of problems with my lungs [asthma]. Perhaps that may be a problem for Sunday, but it is such a great marathon. I enjoy it. Let me do it'.

"So I checked with many people. Most were in favour, though some were critical, suggesting that such a great champion should not do this. But he enjoys it, likes the crowds, likes people. He never could do anything in London, so this is a farewell trip, I guess." Haile aims to go to 30k, preparation for a tilt at the world masters record of 2:08 in Hamburg three weeks from now.

Farah, I suspect, must serve his marathon apprenticeship. He may indeed be a world record contender in the long run, but not yet. Whatever he does tomorrow, it is how he recovers that will dictate whether he races at the Commonwealth Games in Glasgow and/or the European Championships less than a fortnight later. The Kenyan presence at Hampden means there will be no soft ride, but he could well run the 1500m in Glasgow and the 5000m in Zurich.