Some of the world's best athletes will give a very good run for their money in today's London Marathon, others will just pick up their appearance fee and potter round without threatening to win. The world's top distance runners are well rewarded - the best earn one million dollars a year - and they reckon to run only two or three marathons a year.

What a comparison that is to a group of men who can claim - though they never do - to be the greatest, toughest, most committed athletes in the world. They run for no other reward than spiritual enlightenment, hoping to help themselves along the path of Buddha towards a personal awakening. They are the so-called 'marathon monks' of Mount Hiei, Japan.

The monks, known as Kaihigyo, are spiritual athletes from the Tendai Sect of Buddhism, based at Mount Hiei, which overlooks the ancient capital city of Kyoto.

The ultimate achievement is the completion of the 1,000-day challenge, which must surely be the most demanding physical and mental challenge in the world. Forget ultra-marathons and so-called iron-man events, this endurance challenge surpasses all others.

Only 46 men have completed the 1,000-day challenge since 1885. It takes seven years to complete, as the monks must undergo other Buddhist training in meditation and calligraphy, and perform general duties within the temple.

The first 300 days are basic training, during which the monks run 40km per day for 100 consecutive days. In the fourth and fifth years they run 40km each day for 200 consecutive days. That's more or less a full marathon every day for more than six months.

The final two years of the 1000-day challenge are even more daunting. In the sixth year they run 60km each day for 100 consecutive days and in the seventh year they run 84km each day for 100 consecutive days. This is the equivalent of running two Olympic marathons back-to-back every day for 100 days.

Author John Stevens, in his book, The Marathon Monks of Mount Hiei describes the running style which dates back over a thousand years. 'Eyes focused about 100 feet ahead while moving in a steady rhythm, keeping the head level, the shoulders relaxed, the back straight, and the nose aligned with the navel.'

What makes all these distances even more amazing is the manner and the conditions in which the monks run. These runs are usually begun at night and are over mountain paths that are uneven and poorly marked. During the winter months the low temperatures and snow are a great hindrance to the runners. These monks do not wear the latest in footwear and clothing, but run in straw sandals, an all-white outfit and a straw hat. They also run on a diet of vegetables, tofu and miso soup, which modern athletes and nutritionists would deem to be unsuitable for endurance events.

Not only do they wear clothes and shoes unsuited to running, but they have to carry books with directions and mantras to chant, food to offer along the way, candles for illumination, as well as a sheathed knife and a rope, known as the 'cord of death'. These remind the monk of his duty to take his life if he fails, by hanging or self-disembowelment. The course is littered with unmarked graves, marking the spot where monks have taken their own lives. However, there have been no cases of monks' suicides since the nineteenth century.

During theses long runs the monks must make stops at temples of worship that can number up to 260. This means that the 86km run can take up to 20 hours to complete leaving the monk with very little time for recovery or rest, but as an old saying goes: 'Ten minutes' sleep for a marathon monk is worth five hours of ordinary rest.' They also learn to rest sections of their body while running, such as their arms or shoulders.

And then there is the doiri, where the monk faces seven days without food, water or sleep or rest. During this time the monk will spend his entire day reciting Buddhist chants and mantras - perhaps up to 100,000 each day. The only time the monk will leave the temple is at 2am to walk the 200m to a well and return with water to make an offering. He is not allowed to drink any himself and the 200m walk can take up to two hours in the final days of the fast. During his time spent meditating there are two monks who are in constant attention to ensure that he does not fall asleep.

For several weeks before doiri, the monk will reduce his food intake so his body can cope with the fast. The first day is no problem, but there is some nausea on the second and third days. By the fourth and fifth days the hunger pangs have disappeared, but the monk has become so dehydrated that there is no saliva in his mouth and he will begin to taste blood.

The purpose of doiri is to bring the monk face-to-face with death. During this fast, the monks develop extraordinary powers of sense. They talk of being able to hear the ashes of incense sticks fall to the ground and, perhaps unsurprisingly, of the ability to smell food being prepared miles away.

Physiologists, who have examined the monks after conclusion of the rite, find many of the symptoms of a 'dead person'. Monks talk of experiencing a feeling of transparency where everything good, bad and neutral leaves their body and existence in itself is revealed in crystal clarity. Relatives of those who undergo this rite of passage talk of the difference that the seven days makes to those who undergo it. One remarked, 'I always dismissed Buddhism as superstitious nonsense until I saw my brother step out of Myo-o-do [the name of the temple] after doiri. He was really a living Buddha.'

When the Japanese Emperor maintained his court in Kyoto, the monks were afforded a special thanksgiving service in the Imperial Palace after completing their 1,000-day term and the 'marathon monks' were the only people who were allowed to wear footwear in the presence of the Emperor.

Even today thousands will turn out to watch a monk nearing completion of a 1,000-day term, as he runs the old course that now passes through Kyoto's shopping streets and the entertainment district, complete with its bars, restaurants and strip joints. Many turn up hoping to be blessed by these special monks whom they believe have powers to heal.

Japan has the largest number of marathon runners per capita in the world. From the Arctic northern island of Hokkaido to the balmy tropical islands of Okinawa in the Pacific, each and every town will organise a number of long-distance runs and each school will have a strong running club.

There is even a corporate-sponsored running league, whose teams are even allowed to have one foreigner in their team. Jeff Schiebler, a Canadian Olympic runner, is the only non-African foreigner who competes. He described what it is like to run in Japan. 'It is totally different from anything in North America. They have multimillion-dollar contracts, team chefs, great training facilities. That kind of thing makes Japan a power in long-distance running. They go mad for road races. Kids there grow up wanting to be the next marathon champ.'

Japan's love of marathon running was epitomised with the incredible outpouring of emotion that followed Naoko Takahashi's victory in the women's Olympic marathon in Sydney last year. The race and the prize-giving attracted a massive 84 per cent TV rating as the fresh-faced girl from the mountains of Gifu became the first Japanese woman to win an Olympic gold medal.

She became an overnight superstar and her face was splashed across newspapers, magazines and on talk shows. She even received The People's Honour (only the third woman ever to do so) from the then prime minister Yoshiro Mori, who said: 'You have given inspiration and encouragement to youngsters as well as a whole people by crossing the finish line with a refreshing smile.'

Very few runners will cross the finish line in London today with a 'refreshing smile' after 26 hard miles. Grimaces of exhaustion and relief will be a more common sight. However, after looking back at the 26 miles and a bit, there will be a feeling of great personal pride and achievement in their performance. Many will have achieved personal best times and others will have raised hundreds of pounds for charity. But will many of them be able to say they have gained something spiritually, as with the 'marathon monks' of Japan?