They are attempting something that has not been done before. At Ultra Gobi runners have to scramble down and out of steep canyons and cross mountain chains, while the ground – rocks, sharp shrubs and cauliflower-like twisted clods of earth – is a test for people with full eyesight. A lot of the race course will have to be negotiated during the night while navigating using GPS.

American Jason Romero, who is one of only a few blind ultrarunners to have completed a 100-mile non-stop trail race, thinks that the task Leung faces “is brutally difficult”.

As the race approaches, Leung himself remains composed. “I am not afraid of anything. I am not even worried.”

That is because he has faced something far more frightening and dealt with it.

One morning in 2009, his alarm clock rang, but when Leung opened his eyes, there was only darkness. That night, his hereditary degenerative retinal disease had quietly finished its job.

Doctors said there was no cure. A few days later his girlfriend of 10 years left him, completing the collapse of his world.

He got drunk. “I felt like I reached rock bottom. I lost everything – my eyesight, my girlfriend, my job. I was standing in front of the window ready to kill myself.”

He stood there for a long time. The reason he did not jump was that he felt he just “was not done yet”. He had no solution, though, and so continued to drink.

The binge lasted almost two years. “My apartment was full of bottles, takeaway boxes and leftovers. A lot of rats and cockroaches made friends with me,” he says with a laugh.

Two things arrested his free fall – the “admonishment” of newly made visually impaired friends, and the radio.

“I had a habit of going to bed with the radio on – to listen to music or hear the news. Fatal traffic accidents made me think – people die, they exist no longer, they have lost control of their being. I still have myself present, I only lost one organ, my eyes, but I have given up on myself.”

He started to claw his way out of the darkness. “I learned how to walk with a cane, learned Braille.”

One day, Leung went with a friend to a training session organised by the Hong Kong Blind Sports Federation. He “fell in love with running”, despite only being able to manage two laps of the 400m running track. After months of drinking and binge-eating, Leung had a 97cm (38 inch) waist, and weighed 86kg (190 pounds).

That weight is now long gone. Leung’s 10km personal best stands at 38 minutes. His marathon record is equally impressive – three hours 16 minutes – and he often makes top five in able-bodied age categories as well as winning the visually impaired category. His achievements are remarkable for a 51-year-old who had not started running until his mid-40s, eyesight or no eyesight.

Leung’s talent has won him international honours. In 2015 he was crowned champion of the visually impaired group at the Taipei Standard Chartered Marathon. In the same year he finished as runner-up in his age category at one of the world’s most competitive events: the coveted Boston Marathon.

Leung may feel ready for Ultra Gobi. He is certainly fit, running almost 200km per week in training. But it is the guide, Tam, who is destined to be the unsung hero of the adventure should the two succeed.

“My concern is actually not Gary. My concern is for the guide,” says Richard Hunter, programme coordinator at the United States Association of Blind Athletes (USABA) Marathon National Championships and a highly accomplished visually impaired ultrarunner. “I don’t believe there has been a precedent for a single guide to help a blind athlete in an ultra-race with such complex footing and conditions.”

Jason Romero explains that it takes a lot of work to guide a blind runner. “I ran the Leadville 100 [a 100-mile race in Colorado, US] and had two guides – one for 37 miles, the other for 63 miles. They both had trouble doing the work as a guide and covering that much mileage.”

Tam will be the sole guide for the Ultra Gobi’s full 400km, but he is not phased. Very experienced in ultra races, he has trained up on the use of handheld GPS devices, and Gary’s Team has been putting in miles on both trails and roads in Hong Kong.

Guiding blind runner in North Pole Marathon took his breath away

Andy Chik Wing-keung, who in 2014 guided Leung for 19 days on their 1,000km run around Taiwan, has a word of warning for Tam: “Gary is very proud. He will say he is OK whatever happens. He will insist on carrying on. The guide must make him rest.”

Leung says he will give everything he has in the Gobi. “Losing my vision did not mean losing everything. You lose everything when you give up. I experience the world by running and I will keep running till my last breath.”