Ferdie Pachecho knew. Muhammad Ali's fight doctor left the team in 1977 after the legendary boxer had retained his world title against Earnie Shavers at the age of 35. Pachecho's offence was telling Ali it was time to quit.

Ali's next fight, in February 1978, saw him lose to the good but non-legendary Leon Spinks. It was not until three years later, after a final defeat from another non-legend, Trevor Berbick, that the man later named Sportsman of the Century by Sports Illustrated and Sports Personality of the Century by the BBC finally decided to stop boxing.

There are many who believe that Ali's subsequent Parkinson's disease was exacerbated by his decision to keep on fighting, albeit that his victorious battles with Joe Frazier and George Foreman towards the end of his career involved him in a fearsome amount of punishment.

Thankfully, those who earn their living from other sports do not run the risk of serious physical harm if they misjudge the moment to step down, or away.

Haile Gebrselassie, who announced his retirement just over a week ago after pulling out of the New York Marathon, has probably still got a sore knee now. But it will no doubt get better. No amount of miles run is going to diminish this bright, funny, gentlemanly human being as he moves towards a political career that is also likely to resonate internationally as well as in Ethiopia.

Immediately plaudits arrived from around the world for a man who has always behaved with warmth and dignity as well as performing with courage and talent.

Website comments discussed his greatest achievement. Was it the four minute mile which he produced at the end of the Hengelo 5,000 metres when he set his final world record at that distance in 1998?

Was it that unforgettable side-by-side sprint all down the final straight at the Sydney Olympics two years later which eventually saw him take the 10,000m gold from that other talented and dignified figure, Paul Tergat of Kenya, by the margin of nine hundredths of a second.

Personally, I think the way he handled himself as he sought a third consecutive 10,000m title at the Athens Olympics four years later could be seen as his finest hour. Knowing that an Achilles tendon injury had crucially curtailed his preparations, he went to the front just to see if he could break clear of the young compatriot Kenenisa Bekele, who had already started to better his record times and had beaten him to the world 10,000m title in Paris the previous year.

Of course, he couldn't. And of course, he was the epitome of sportsmanship as Bekele claimed his first Olympic title.

But, as the 37-year-old double Olympic champion and marathon world record holder has since indicated, he is Having Doubts. Indeed, reports coming out of Ethiopia today indicated that the runner whom another double Olympic champion, Seb Coe, described last week as "the greatest athlete of the last 50 years and arguably the best of all time" has thought better of his emotional statement in the wake of his New York disappointment.

According to the website EthiopiaFirst.com, Gebrselassie announced that he will return to running when he gave a speech at the Great Run in Nekemte, a fundraiser for the Wollega Stadium. The report was confirmed by Ethiopian Television Sports and another website, EthioSports.com.

When, tearfully, he announced to journalists in New York: "I never thought about retirement. But for the first time, this is the day," his memory was playing him up a little. Gebrselassie's manager Jos Hermens recalled this week that after pulling out of the 2007 London marathon with breathing difficulties, the multiple world record holder had told several people that he was thinking of quitting, adding "I will never learn the marathon. I'll never make it."

As Hermens added, Gebrselassie subsequently set two more world records.

Gebrselassie is far from being the first great sportsman to misjudge the time to give it a rest. Another classic instance occurred at the 1996 Atlanta Olympics, where Steve Redgrave, having just secured his fourth rowing gold at the age of 34 – in company with Matthew Pinsent in the coxless pair - announced to the BBC television camera: "Anyone who ever sees me go anywhere near a boat again, ever, you've got my permission to shoot me."

Four years later in Sydney Redgrave collected gold medal number five...

Redgrave took a risk in going for that final title – and had to show almost superhuman resolve to secure it as he suffered from both diabetes and colitis in the run-up to the 2000 Games.

Every now and again, we see a faultless career, pursued to its heights, relinquished at its peak, with no regretful memories of failure.

Such was Herb Elliott's career. This native of Perth broke world records for the mile and 1500m in August 1958, having won gold in that summer's Commonwealth Games 880 yards and mile.

Two years later the young man trained by the visionary Percy Cerutty – "Faster," he told Elliott as he sprinted up the sand dunes at Portsea. "It's only pain" - won the 1500m at the Rome Olympics in a world record of 3min 35.6sec.

Elliott then began a degree at Cambridge University, and retired in 1962 after winning the half mile in the match against the Amateur Athletic Association. He had broken the four-minute mile on 17 occasions, and bowed out of the sport unbeaten at either the mile or the 1500 metres.

Perfect. But Elliott was only 24. Who knows what heights he might have gone on to attain had he continued. Who knows how he might have fared at the 1964 Tokyo Olympics against Peter Snell, the Kiwi who won the 800/1500m double in Japan before retiring almost as precipitately as Elliott the following year at the age of 26?

We won't know. And that disappoints.

We always want our heroes to run that final risk, to seek one more crowning glory. And yet when – and here the experience of Ali is emblematic – they seek that final flourish beyond the years their talent allows, our memory of them is diminished by regret.

Who knows how it will turn out with Gebrselassie? If he does return, he may never win another race again. But there is a strong sentiment in favour of his continuing, and trying his luck at the London 2012 Olympics. It is hard to see any failure having a serious effect on the reputation he has already established in world sport – so why not?

Mike Rowbottom, one of Britain's most talented sportswriters, has covered the last five Summer and four Winter Olympics for The Independent. Previously he has worked for the Daily Mail, The Times, The Observer, the Sunday Correspondent and The Guardian. He is now chief feature writer for insidethegames