Since adolescence, I have decided that a spartan lifestyle with minimal luxuries and plenty of discipline would help me get through life without hankering for riches, fame, power and other distractions Buddhists dismiss as "red dust".

The two "luxuries" I allowed myself were exercise and hiking. Exercise was initially not so much a luxury but more to train discipline. But, over the decades, exercise has become so much a part of my daily life that it is now almost a necessity.

It has indeed trained my sense of discipline, but that discipline has become so extreme that, many a time, I have used mind over matter to achieve my own target.

But later, I have suffered the penalty of severe fatigue, which sometimes lasts weeks on end. After such a long period out of action, I then have to build up my fitness level again very slowly. Not infrequently, I push myself hard sooner than I should, and I slide back and have to start all over again.

This year has been the worse so far, and there is a medical explanation. For marathoners, the deterioration with age often begins to affect results shortly after age 30. It continues at a steady pace through their 50s. At about age 60, the decline accelerates.

Dr Ray Fair is a professor of economics at Yale best known for devising a mostly accurate formula to predict winners of presidential elections. He has also completed 17 marathons and counting.

He studied the question: How can one keep racing against oneself as age slows one down? He wrote a research paper studying world records for runners all the way up to 92 years old, and has developed tables that try to track the body's physical deterioration and set an ever-moving target.

The records show that the world's best runners lost just a minute or so a year in their 40s when running a marathon.

Little wonder that up until my late 50s, mind over matter worked. It took a little more effort to stay where I was, but I was not deteriorating much. In fact, at the age of 47, after a series of operations left me extremely debilitated, my doctors, physiotherapist and sports trainers put me through a rigorous training programme that included strength, balance and agility training - over and above my obsession of endurance aerobic exercise.

The test of my physical improvement was when I hiked alone through the Haleakala Crater on Maui. At the age of 48, I completed the strenuous hike, which 18 years earlier took me 10.5 hours to finish. This time, I finished it in eight hours. One of my sports trainers, who was an ex-commando, said to me: "You have passed your commando course." I was jubilant and believed that mind over matter would always work for me.

But I turned 60 in January, which is approximately the age when a sudden decline occurs. No wonder that, since last year, I have been struggling unsuccessfully to maintain my previous endurance and speed.

This year, I conceded defeat to age and accepted that mortal rules now apply to me. No matter how determined I am, the body cannot perform as it did before.

I have never run a marathon, but my exercise regime used to include a 15km run up and down a hill, then a 4km swim, and water running for 0.5km every day.

I felt that was almost equivalent to the effort of running a marathon, and so compared myself with marathon runners. It is interesting that middle-distance runners deteriorate faster at first, but they do not slow down as much as marathoners in their 60s and 70s.

That explains why my father's security officer, whose fastest time in the 2.4km run was eight minutes in his 20s, now needs 8.5 minutes. But he already notices that he takes longer to recover from a strenuous workout. He advised me to reduce the intensity or the amount of my workout, or both, and I now reluctantly agree that that is sensible advice.

I am a neurologist and don't make a living by exercise. Why am I so fixated on my fitness level? Because determination could overcome the ravage of age for much of my life, I lulled myself into believing that some mortal rules don't apply to me. But not any more.

Whether or not I like it, I need to accept the inevitable. With respect to many things which mean so much to normal mortals, my attitude is na de qi, fang de xia, which in English means: What I can pick up, I can easily give up.

But now letting go of something that means little to most people is very difficult for me. But I have no choice but to adapt. I cannot even blame fate because we are all fated to grow old and less vigorous.

Age has taken a toll on my appearance. I have white hair liberally scattered among what still remains a black background. My face has aged so much that I look not just plain, but actually ugly.

All that I accept with no distress. But it is my deteriorating physical prowess that upsets me. Nonetheless, I have started to adapt.

My exercise regime now is 6.5km of running and 6.5km of brisk walking, swinging a 1kg dumbbell in each hand. A time will come, probably months down the road, when I have to cut back further.

I have stared old age in the face and, finally, accepted the inevitability of physical decline, which will continue until I die. Perhaps, mental decline will set in some time in the future.

I will still try my best to maintain both my physical and mental abilities. But I will only accept the decline as inevitable when it is obvious that, however hard I try, I cannot retain my younger physical and mental state.

I am exceptionally determined (or stubborn), and I don't give up easily. So, I see a long battle ahead.