Sichuan-Tibet section of the National Highway 318 features dangerous winding mountain roads and 12 peaks that rise 4,000 meters above sea level.

But a 36-year-old man from Sichuan province and his 12-year-old son managed to walk its length in 40 days to reach Lhasa, capital of the Tibet autonomous region.

Zhang Wei, a native of Zigong, said he had planned the trek for nearly a year, taking his son's health into consideration.

"I had talked to my son about the hardships on the journey and gained his consent before we started on July 8," Zhang said.

One month before starting out, they made a detailed plan for each day, deciding what to wear, what tools, medicine and instruments to take, when to start each morning, the route to take and where to put up for the night.

They drank rhodiola rosea, a Tibetan medicinal herb believed to prevent and mitigate altitude sickness, and ran every day in order to improve their lung capacity.

On July 8, they took a bus from Zigong to Kangding in the Ganzi Tibetan autonomous prefecture, where they started walking along the Sichuan section of the National Highway 318.

Built from 1950 to 1954, the National Highway 318 starts in Shanghai, passes through Jiangsu, Zhejiang, Anhui and Hubei provinces, Chongqing, Sichuan and ends in Tibet. With a length of 5,476 kilometers, it is the longest highway in the country.

At 5 am the next day, they started to cross Zheduo Mountain, 4,298 meters above sea level, with other hikers. Although Zhang and his son suffered altitude sickness, they dared not stop for fear they might not reach the campsite where they planned to sleep before it got dark.

Despite using drugs to battle against the sickness, they were so tired they lagged behind the other hikers and only managed to reach the campsite at 8 pm.

Frustrated, Zhang asked his son if they should stop. He said it was a great feat to cross Zheduo Mountain and they could return to Zigong without losing face. But his son said they should continue the journey.

The route from Kangding to Lhasa is 1,700 kilometers. Although it was summer, they experienced several different seasons in a single day. It was as cool as spring in the morning, as hot as summer at midday and as cold as winter in the evening.

Altitude sickness can make a hiker without any luggage feel as if he is carrying something heavy on his back.

Zhang had a backpack weighing 15 kilograms and his son had one weighing 10 kilograms. Each day they walked more than 30 kilometers.

Because of the hardships, Zhang said they wanted to give up every day, but always carried on the next day.

On Aug 17, the pair reached Lhasa, where they stayed for 10 days, visiting the Potala Palace, learning how to make Tibetan incense and visiting artists making thangka paintings (traditional Tibetan Buddhist art). On Aug 26, they flew from Lhasa to Chengdu.

Marveling at their impossible mission, some friends thought Zhang was mad taking a 12-year-old boy to Tibet on foot.

Yet the father said the purpose of the journey was to teach his son to be brave and endure hardships in life.

It was not the first time the pair had undertaken a long journey. In the summer of 2015, Zhang and his son went walking and climbing mountains in Yunnan province and the Guangxi Zhuang autonomous region for 40 days. The next year, they went hiking in Guizhou province.