Most runners consider the treadmill a second choice, reserved for times when the weather is bad or they don't have much time. But other than his school's annual sports day, jogging indoors on a moving belt is the only way to work out for partially sighted marathon lover Tse Ka-wai.
The 17-year-old, who goes to Ebenezer School and Home for the Visually Impaired, has no problems catching the right bus, recognising faces and reading books, as long as he leans in.
But when he picks up speed, he struggles to detect obstacles such as uneven ground, puddles or other runners. "It would be too dangerous to run outdoors alone," Ka-wai says.
With the help of guides who volunteer for the Marathon 101 "Stand by You" Training Programme, he is taking part in an event at the Standard Chartered Hong Kong Marathon for the second time.
On Sunday, he will run the 10km race with his sighted partners, Form Five student from La Salle College, Frederick Li Yan-chak, and Chu King-long, another Form Five student from Cheung Sha Wan Catholic Secondary School.
The trio are among 22 visually and intellectually impaired students and their 37 guides who teamed up and trained for eight weeks before the popular annual event, which attracted 73,000 runners this year.
Ka-wai's guide Frederick, who has taken part in the marathon for three years, did not have any previous experience working with the visually impaired. He was under the impression his partner would be reserved, but found Ka-wai "very talkative and positive".
During training, Frederick became aware of his partner's difficulties, slowing down to match his pace. He would also run in the shade to help his light-sensitive teammate, or remind him to don sunglasses.
They train at the safe and well-paved Happy Valley Sports Ground, but will need to be on full alert on Sunday while running on roads alongside thousands of others.
"It's important that we guides take the lead and warn of hazards along the way," says Frederick, "because we will be running in an unfamiliar environment, and we need to avoid being bumped into and bumping into others."
Frederick has benefitted from running with Ka-wai. "When I ran by myself, I found it hard to sustain the same pace over long distances," he says. "When running with Ka-wai, I can better maintain a constant pace."
They hope to complete the run in an hour.
Fellow guide Ronald Lam Chin-ching, also a Form Five student at Cheung Sha Wan Catholic Secondary School, teamed up with an autistic student from Hong Chi Winifred Mary Chueng Morninghope School. He has been running long distance for five years, but this is his first race for his partner. It is also the first time he's got to know people with intellectual disabilities.
"My partner is very persevering and bright," he says. "He is good at geography and interested in numbers - if you give him a random date, he could tell you which day of the week it is in three seconds."
Even though Ronald's partner doesn't enjoy stretching, the pair spends half their time on the exercises to help avoid muscle cramps. Ronald also has some tricks to motivate his partner.
"It always works when I threaten to leave or not show up next time," says Ronald. "I also encourage him to speed up so he can beat other runners."
Ronald says they should finish the race in an hour and 15 minutes, and is confident they will make the finishing line.