By this time next week, Mo Farah will have run his last 10,000m race. The following weekend he will end his track running altogether by defending his world supremacy over 5,000m.

And Usain Bolt will end his time as the fastest sprinter in history.

Two extraordinary individuals doing the simplest thing better than anybody else on planet earth. Two runners who beguile us with their God-given genes and their superlative competitive spirit.

They transcend a sport that others have so corrupted that even the IAAF president, Sebastian Coe, admitted to the BBC: "I cannot guarantee the world athletics championships will be drug-free."

Let us pray that Farah and Bolt are above that kind of thing.

Now, before I go on, who among you plans to watch their final races in real time?

Geographical time zones are against you. The championships at the London Olympic Stadium will largely be contested in your sleep. At least Mo and The Bolt do their thing at the weekends - prime time for viewers in Europe, either the middle of the night or a ridiculously early breakfast call where you are.

We have delayed broadcasts, and we can all record broadcasts at a whim. But nothing beats the drama of seeing things as they happen.

Waking up, risking that someone, somehow will break the suspense of whether time or man has eclipsed the two great champions in their farewell runs takes away the wonder of it.

And both Farah and Bolt are pushing their barriers. They have known this past decade how to prime their bodies, and minds, to a certain time on a certain day. They do not have eternal powers; they are retiring from their life-defining events because their body is telling them their time is not infinite.

To cut to the chase, Mo Farah is scheduled to start his 10km track race at 4.20am next Saturday.

Usain Bolt should be on his blocks in the 100m final at 4.45am on Sunday, Aug 6.

And then, provided nothing amiss takes place, Farah's final run in the stadium should kick off in the 5,000m race at 3.20am on Sunday, Aug 13.

Am I reducing the world championships to two athletes? Yes, guilty.

Can I think of anyone, male or female, to rival the impact these two individuals have on the multi-faceted sport of athletics? Well, no.

That is no slight on the boys and girls striving to win their specialist events at these championships. Nor are we discussing here the controversial issue of Caster Semenya's testosterone levels, or the question that has challenged sports ever since Richard Raskind sought to compete as Renee Richards in women's tennis in the 1970s.

If I were to hazard a guess where this is going, I can imagine America pushing for a World Transgender Games, possibly within a decade.

But Bolt and Farah are simply racing against time, and the inevitable deterioration of form. Bolt will be 31 next month, and he looks and sounds just a little weary, even vulnerable.

His 100m world record run was eight years ago in Berlin. The men who want to take him down do not have to beat his 9.58 seconds time, they simply have to beat him on the track.

Farah is different. For a start, he isn't retiring. He will finish in the arena and, at 34, switch to road running as other great distance champions have done in the marathon.

In fact Farah has pushed the boundaries without ever being a world record-holder against the clock. His 12min 53.11sec lifetime best over 5,000m ranks 64th in the history of the event.

Kenyans and Ethiopians by the handful plot to take his Olympic and world gold-medal runs from him in both disciplines. He beats them, or has up to now, by being the master of race tactics.

He has incredible power-to- weight ratio in his deceptively lightweight 60kg-frame. And he possesses a cunning, inner sense of when to release his breathtaking sprint to the line.

Farah says he is nervous about what he has left for the London championships. He admitted recently that his body is not responding in quite the same way to a 193km-per-week training regimen as it did a few years ago.

Such utterances of vulnerability might simply goad his opponents. Or they might be Father Time calling upon Farah.

He is the second-richest athlete given that Bolt and he are on the Forbes list for accrued millions from sponsorships, endorsements and appearance money.

Every year, boffins around the world spend their time in laboratories poring over the genetics that make Bolt so fast.

The best answer in quite some time came from Bolt's agent, Ricky Simms, when he was asked (again) for a comment from his runner to the latest sports science theories in the US. "Usain," he said, "isn't the kind of person who studies this type of thing."

Farah, by contrast, is immersed in American theory. His coach Alberto Salazar runs the Nike "project" in Oregon. The project claims to be a world centre for biomechanics, but it is being investigated over doping claims.

Farah denies ever taking drugs, as does Bolt. We have to give them the benefit of the doubt. We should not taint them by association because of those they train with.

If we enjoy anything in athletics, it is their story. Bolt is a freak. Farah is complex - born in Somalia, living in America, running for Britain where he was granted refugee status at eight.

At times when he looks so indomitable, we think of his twin brother Hassan. They lived and played together, read the Koran together before their father had to choose three of his six children to take to England.

Hassan, a telecommunications engineer in Somalia, said a year ago: "Sometimes when we raced Mo won, sometimes I won. He's my brother, and I love him, but who knows, we could have been famous Olympic twins."

Who, indeed, knows much about anything in this life?