Nothing beats this challenge. Below is an excerpt from an article on Wikipedia. The whole article may be read here.
Only 46 men have completed this challenge since 1585.
The Kaihōgyō (回峰行) is a set of the ascetic physical endurance trainings for which the Japanese ‘marathon monks’ of Mt. Hiei are known. These Japanese Monks are from the Tendai school of Buddhism, a denomination brought to Japan by the Monk Saicho in 806 from China.
Part of Tendai Buddhism's teaching is that enlightenment can be attained in the current life. It is through the process of self denial that this can be achieved, and the Kaihōgyō is seen as the ultimate expression of this desire.
There are many serving priests at the Temple on Mt Hiei, but very few of them have completed the Kaihōgyō. Many who have completed it come from outside of the Order.
The selection process for the Kaihōgyō is after the first 100 days of running, the Gyoja (trainee Monk) will petition the senior Monks to complete the remaining 900 days. In the first 100 days, withdrawal from the challenge is possible, but from day 101 onwards the Monk is no longer allowed to withdraw; he must either complete the course or take his own life. The mountain has many unmarked graves from those who have failed in their quest, although none date from the 20th/21st century.
The ultimate achievement is the completion of the 1,000-day challenge, which would rank among the most demanding physical and mental challenges in the world. Only 46 men have completed the 1,000-day challenge since 1585.
The Kaihogyo takes seven years to complete, as the monks must undergo other Buddhist training in meditation and calligraphy, and perform general duties within the temple.
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.'