In physics, Bill Taylor showed me that the little things teachers do can stand you in great stead for the rest of your life. One day we were all nervously waiting to start a mini-exam. He said: “You have 40 minutes. Begin.”

The instructions at the top of the question paper began by saying “You must answer 10 questions”, but the last line said “Ignore everything you’ve just read. Fold your arms. Don’t do the paper.” I did that, but a lot of the others just dived in without reading the last line and answered the questions. It was a great lesson about the need always to take a few minutes to prepare yourself.

I don’t think we were taught to enjoy the subjects so much as how to pass exams. In French, Mr Haynes was brilliant at exam preparation: there were 22 As, including me, and two Bs in our class at O-level.

I vividly remember the first time our excellent maths teacher, Andrew Jarman, who’d been a pupil at the school, came into our classroom; he just connected with us straight away. For one year Mr Jarman made everything clear.

The next year, instead of him, we had a teacher who was so bad that we just sat there in a daze. Teachers are under-appreciated, but there’s no excuse for being a bad teacher - you’ve got people’s lives in your hands.

For A-level, I chose maths, chemistry and biology. My father was a doctor, and I didn’t want to be a vet or a dentist, so I thought I might as well be a doctor, too. A lot of people felt that was a tall order, but once I set myself a goal it’s in my nature to achieve it.

My offer to read medicine at Bart’s Hospital in London was three Bs. But the problems caused by that poor teacher contributed to making the day I took maths A-level the worst - and at the same time the best - day of my life.

I started doing one of those questions where you are given an answer and have to show how you reach it. I knew what to do, but couldn’t get the working right. I tried again and again - and then just lost it. Nothing has ever come close to the panic I experienced at that moment - no Olympic final, no World Championship final, nothing. I only finished about half the paper and walked out knowing I wasn’t going to get the B grade I needed.

I got an A in biology, a B in chemistry and a D in maths. Bart’s rejected me. If I’d got three Bs I would’ve gone straight to university, I’d never have taken up athletics, I’d be a doctor and no one would have heard of me. Instead I took the year off and it changed my life.

At school I’d been selected for the England Schools Championship. The local athletics club used to knock on my door, but I didn’t like individual sport. At the start of that year off, a friend of mine who had run for Hampshire persuaded me to give it a go. I joined Southampton Athletics Club, and within two months I was running for Great Britain.

I also went for private tuition with a maths teacher called Mr Cole. In two or three sessions a week over a couple of months he made everything clear again. I sat the A-level again, got a B and was accepted to read medicine at Southampton University. But after the first term I realised it was impossible to sustain both the athletics - I had become European Junior Champion -and the demands of medicine, so I left.

It’s extraordinary to trace my career back to that maths exam. The message I take from that experience is one I repeat when I do motivational speaking for business people: when something bad happens to you, you have to step back and ask “What good can come from this?” At 17, I couldn’t do that; I came out of the exam thinking the whole world had ended.

Part of me would love to be a teacher, but I’m in a unique position with my motivational work in that I can influence people without teaching full time. The skill of the great teacher is not taking a Roger Black and making him an Olympic medallist, it’s taking the ones with less talent and getting them to fulfil their potential.

* Roger Black, MBE, 32, won silver in the 400 metres at the 1996 Atlanta Olympics and the 1991 World Championships. His autobiography, ‘How Long’s The Course?’, is published by Andre Deutsch on June 28. He was talking to Daniel Rosenthal