If you are a professional marathon runner, this column does not apply to you. Or perhaps it does? One can’t be sure without the experience of being a professional marathon runner. But for those who occasionally go for a jog, and know what an effort it takes, and how much it can hurt, the analogy applies.

Imagine you have been earning a living running marathons for 20 years. They never got any easier. Once in awhile, there would be a race on a course which was largely flat and, with luck, the wind was behind you. But it was still 42.2 kilometres. And you still had to pound the pavement for every metre. The ankles, knees and hips still ached afterwards.

But mostly the races were run up steep hills, across rough terrain and in conditions designed to stretch and test your physical, emotional and psychological limits. The organisers chose those things because they usually meant for good viewing.

You trained hard, not just the same old routine, but encouraged and accepted new theories on how best you could improve. But as the marathons mounted up, the aches and pains grew increasingly hard to recover from. You loved your work and everything which came from your success, but there was a time approaching when you knew enough would be enough.

It’s often impossible to tell when that time would arrive. Would it be a gradual realisation or a lightning bolt of mental clarity? Or an injury? What if that moment arrived in the middle of the Olympic Games, with the entire world watching? Would you step aside in the middle of the race, walk meekly onto the pavement and bow out?

Does pride play the ultimate role or practicality? Or do you hope to wake up one day and say simply: “I’ve run my last marathon?”

Of course, as a celebrated marathon runner, there will be many opportunities to run shorter races, social races and charity events. Once a runner, always a runner. No athlete has ever thrown away his shoes and said: “never again.”

That moment of clarity was experienced by Jacques Kallis during the test series against India. Test cricket is the marathon of the game. His fitness, skills and physical attributes (eyes!) were as good as they had been 20 years earlier, but the mental aspect was just below par.

Could he have played another test match? Yes. Did he really want to? Yes, but not as much as he had wanted to play the previous 165.

And that was that. For the last five or six years, Kallis believed that the key to his retirement would be the moment when he did not wake up before a test match feeling there was nothing else he would rather do.

His best friend, Mark Boucher, had said for years that there was “nowhere to hide” in test cricket and Kallis knew that was the truth. “If you are not 100%, physically and mentally, then you will be caught out,” Boucher always said.

It may only have been five or ten per cent, but when Kallis felt his commitment had been compromised, he withdrew.

As will become apparent in the months ahead, however, his commitment to becoming a specialist at running shorter races is extreme. He is not resting on his many laurels. He has spoken of reinventing himself as a one-day player in his bid to earn selection for the 2015 World Cup squad.

We have much to look forward to from the former marathon runner.