Eddy Merckx, five-times winner of the Tour de France and acknowledged by many of cycling's cognoscenti as the most accomplished rider the world has ever known, described it as "the hardest ride I have ever done."
A fortnight ago Australia's professional rider and double gold medallist at the Glasgow 2014 Commonwealth Games, Jack Bobridge, described it as "a bit like dying."
What are these two champions talking about? Challenging for the world hour cycling record, that's what.
Back in 1972 the fabled Belgian achieved his goal at an altitude of 2,300m in Mexico City's 1968 Olympic velodrome, covering 49.431km.
Bobridge, racing in Melbourne, fell an agonising 500m short of his target - the mark of 51.852km (officially adjusted) set by Austria's Matthias Brändle at Switzerland's Aigle Velodrome on October 30 last year.
And now what has long been established as the blue riband mark in world cycling has been claimed by another masochistic challenger, Rohan Dennis.
Bobridge's 24-year-old Australian team mate set off in pursuit of Brändle's mark on the same track in Grenchen, Switzerland which staged the previous world hour record of 51.110km, with which Germany's 43-year-old Jens Voigt marked his retirement on September 18.
Dennis' personal mission was successful as he crossed the line with a new mark of 52.491km. He hoped the record would last "for a little while." But he knows that it will already be in the sights of other top class riders, not least the English pair of Alex Dowsett, winner of the individual time trial at the Glasgow Commonwealth Games, and the illustrious Bradley Wiggins, multiple Olympic champion and the first Briton to win the Tour de France.
Dowsett had planned to attempt the record at the Lee Valley VeloPark used for the London 2012 Olympics on February 27, but has had to postpone his effort after taking longer than expected to recover from a broken collarbone.
"I'm really gutted," he said. "I've never worked harder for anything and to lose this opportunity is a blow. The positive is that we will be attempting it at a later date."
Wiggins is planning an attack on the record some time in June, most likely on the Lee Valley track.
And in the meantime, the women's world hour record is also under threat from a Briton - multiple Paralympic champion Dame Sarah Storey has said she will target the existing record of 46.065km set in 2003 by Dutch rider Leontien van Moorsel.
Dame Sarah will make her attempt on Saturday, February 28 at the Lee Valley VeloPark during round five of the Revolution series.
All of a sudden, it seems, there is massive excitement and anticipation being generated by this test which was first recorded as being successfully carried out by American Frank Dodds when he covered 26.508km in the allotted time back in 1876 at the Cambridge University Ground.
There is a reason for this - and it has to do with the International Cycling Union (UCI) changing its mind. Or changing its mind back again.
Dodds rode a penny farthing. Imagine how nonplussed he would have been had the UCI of the time (OK there wasn't one until 1892, but bear with me) come back to him a few years later and said: "Great ride, Frank. But sorry, your mark's scrubbed. We've decided the record only counts if it's been set on a traditional bike. That means, a velocipede."
Happily for Dodds, no such ruling ruined his sense of achievement. When he lost his record, it was for the very good reason that someone else had gone a lot further in their 60 minutes.
Herbert Lydell Cortis managed 32.454km in 1882. A decade later, after the foundation of the International Cycling Association (which was subsumed into the UCI set-up in 1900), the first official record was established when Henri Desgrange - a man whose only other claim to fame was that he established the Tour de France - covered 35.325km on May 11, 1893 in Paris.
But more than a century after Dodds' high achievement (those penny farthings were tall as giraffes), a handful of his illustrious successors found their own records being downgraded through a modern variant on the "penny-farthings no go, has to be a velocipede" adjudication.
In 2000 the UCI reacted to a growing sense of unease at a succession of records set by Italy's Francesco Moser, Britons Graeme Obree and Chris Boardman, Miguel Indurain of Spain and Tony Rominger of Switzerland employing innovative technology (disc wheels, customised handlebars) and stances.
None was more innovative than Obree, who broke the record in 1993 and, after Boardman had eclipsed that mark, 1994, riding on a bike nicknamed Old Faithful he had built at home and which included parts from his washing machine.
Obree, who suffers from bipolar disorder and has attempted suicide three times, also devised a novel posture as he rode with his hands folded back under him, giving rise to the description "preying mantis" handlebars.
And what the UCI said was that all men's records since Merckx's 1972 mark, achieved on a regulation drop handlebar bike with round steel tubing frame and wire spokes, had to be degraded to Best Human Efforts.
The women's record reverted to the 1978 effort of 43.086km by Cornelia (Keetie) van Oosten-Hage, which meant annulling the 48.159km achieved by Jeannie Longo in 1996.
Official UCI records had henceforth to be set with equipment broadly similar to that with which Merckx and Van Oosten-Hage had succeeded. In other words, no aerodynamic time trial helmets, no discus or tri-spoke wheels, no aerodynamic bars or monocoque frames.
Suddenly it was a different game. It was as if you had said to the Premier League that they would have to start playing with laced up footballs, otherwise their goals would not count.
Boardman's existing record of 56.375km, set on the Manchester Velodrome in 1996, thus became the Best Human Effort.
Not surprisingly, perhaps, the swift records and record responses - with Obree and Boardman coming on like Steve Ovett and Sebastian Coe - ceased.
Later in 2000, eight years after he had taken Olympic gold in the Barcelona Games time trial, the 32-year-old Boardman - riding a Merckx-style traditional bike - established the "new" record on the same Manchester track, this time covering 49.441km.
Which effectively put a stop to the progress.
It was not until five years later, in Moscow, the Czech Republic's Ondřej Sosenka managed to reach 49.700km as the final second was ticked off the hour.
The credibility of Sosenka's achievement was seriously undermined given that he suffered a second, career-ending doping suspension in 2008.
But it was his mark which was the official target in May last year when the UCI decided to un-change its collective mind and allow innovative technology to become a part of record efforts once again.
Effectively, there is now a unified classification which makes aero helmets and modern day track pursuit bikes, with triathlon handlebars, disc wheels et al, eligible for record purposes.
And so the interest has surged again in a record previously held by legends of the sport such as Fausto Coppi and Jacques Anquetil, with first Voigt and then Brändle making their marks in the space of six weeks.
Wiggins reaction upon hearing of the rule change last year was to say that he would fancy having a shot at the record. But, classic Wiggins, he added: "It kind of begs the question, 'why did they change it in the first place?' We've lost a decade now of the hour record.
"It's a shame, really, that we've missed maybe [Fabian] Cancellara doing it five or six years ago. So it's good I guess that they've gone back now."
At the time, the new UCI President Brian Cookson said: "This new rule is part of the modernisation of the UCI Equipment Regulation. Today there is a general consensus that equipment used in competition must be allowed to benefit from technological evolution where pertinent.
"This kind of evolution is positive for cycling generally and for the Hour Record in particular. This record will regain its attraction for both the athletes and cycling fans."
Cookson subsequently welcomed the news that Dame Sarah would attempt the women's record. "After two UCI Hour Records in quick succession for the men, I am very pleased the existing women's record will now be challenged," he said.
"Dame Sarah Storey's attempt will be eagerly awaited and I am sure it will prompt other top women riders to try to claim this prestigious record."
Now, for the moment, the men's record is being stalked by Australia - through Bobridge and the new man at the top, Dennis - and England - through Dowsett and Wiggins.
There is a faint echo of that other Anglo-Australian rivalry over a Blue Riband record - the Four Minute Mile, to which both England's Roger Bannister and Australia's John Landy aspired before Bannister got his telling run of 3min 59.4sec in first, to historic effect, on May 6, 1954.
Wiggins, who said he had "never really considered" attempting the hour record before the May 2014 reversion, concluded: "I'd like to do it. For me, the last person to do it in that position was Tony Rominger and Indurain. And I've always said that I'd love to go for it just to compare myself to Indurain purely over an hour. So I would consider it now, actually."
Indurain set an hour record of 53.040km in 1994, beating Obree's mark at the time. Rominger then broke the hour record twice in 1994. He used the Bordeaux velodrome to ride 53.832km and then 55.291km.
Last September, Voigt's swansong sharpened the edge of Wiggins' ambition.
"I want to have a go at the one hour record next year after what Jens did last week," Wiggins told BBC Sport. "I was a bit surprised by Jens' decision to do it but what he did was fantastic and fully deserved.
"I could just go and do it next week but if I do it, I'll only do it once."
If Wiggins does do it only the once, no less an authority than Merckx believes he cannot afford to fail.
"If you're nobody and you don't beat it, there's no problem, but if Wiggins attacks the Hour Record, he has to beat it," he told Cycling Weekly.
"It was the same way with me because I'd already won the Tour de France two or three times, the Tour of Italy, the classics and things like that. The journalists in Mexico thought I couldn't beat it. I knew that I had to if I went for it. I also felt that my career wouldn't be complete if I didn't break the Hour Record."
Merckx added: "Wiggins should break it, though. For sure. Although I think 55 kilometres is too much. It is too much in one hour, even for Wiggins, who's an exceptional athlete."
The Lee Valley track on which Wiggins - who will turn 35 on April 28 - is reportedly considering using to achieve another historic flourish in his career runs for 250 metres.
Merckx, now 69, says that if he were to attempt the hour record again he would choose to do it in a closed velodrome, but would seek a track like the one in Moscow, on which Sosenka set his mark, as it is 333 metres round.
"The bigger the track, the better it is for pushing through the curves," said Merckx.
Right now, however, it seems Switzerland is the hot venue for this record.
Dennis rode on a modified BMC Time Machine to meet UCI standards when he took the record on the Grenchen track today, using fixed gearing to aid his efficiency.
His success meant another spin of the wheel for one of cycling's most compulsive areas of competition. The quest to earn the world hour record is clearly undergoing a revolution. And it seems Wiggins is biding his time to take his turn.
Mike Rowbottom, one of Britain's most talented sportswriters, covered the London 2012 Olympics and Paralympics as chief feature writer for insidethegames, having covered the previous five summer Games, and four winter Games, for The Independent. He has worked for the Daily Mail, The Times, The Observer, The Sunday Correspondent and The Guardian. His latest book Foul Play – the Dark Arts of Cheating in Sport (Bloomsbury £12.99) is available at the insidethegames.biz shop. To follow him on Twitter click here.