"Before I started training for snooker, I thought it was an easy indoor sport. I imagined I could take breaks before each shot and there was no need to hurry," On-yee says.
"But, I soon found out that, although the sport doesn't require much speed on the table, we need to stay focused for many hours and plan our game strategy, deciding which balls to approach and how to hit them.
"I wasn't happy with my weight and standing for a long time took more energy than if I'd been lighter. So I decided to start running and burn calories."
On-yee had this plan in mind for a long time, but it only became a reality when her senior teammate Ivan Chan Kwok-ming asked her to join him in the Standard Chartered Hong Kong 10km race last month.
"Ivan wanted our team to start running to help us keep fit," On-yee says. "[It was just after the Asian Games and] I told him I wasn't ready as we had less than three months to prepare. But he persuaded me to take part anyway."
The hardest thing for On-yee to deal with was her snooker coach fretting about injury.
"My coach was afraid I would hurt my leg if I ran too fast, so he was always reminding me to slow down or even walk in training - and even in the race," On-yee laughs. "Sometimes I think we'd be better off walking so my coach wouldn't have to worry."
When she started running, On-yee would take to the streets near her snooker club in Sheung Wan and run around the Happy Valley Racecourse.
"I felt so tired ... but when I got used to jogging, I started to have a fresh mind and body that I've never had before. I could stay focused for longer and I felt like I had less pressure than before," On-yee says.
On-yee and Ivan became quite famous in the run-up to the marathon because they attracted a lot of media attention.
The media reports had an inspirational effect on the sport.
"All these reports help to build a positive and healthy image for our team. In the past, parents didn't want their children to stay late at snooker clubs because the government placed restrictions on kids under 16 playing at clubs after 8pm," On-yee says.
"People thought snooker clubs were dangerous places for their kids. Now, we have more youngsters in the sport, and we can see how popular the new youth tournaments are. Snooker is even becoming more popular among girls."
Although more children are interested in snooker, all cue sports were dropped from the Asian Games after last year's Guangzhou meet, so On-yee will not be able to defend her team title or try for an individual gold in 2014. But she says she focuses on each match rather than targeting a big tournament.
"I enjoy the moment I improve in my skills and strategy. That is the basic requirement of every match and when I play well in a tournament, I get closer to the prize and trophy," On-yee says. "Thinking this way means less stress, and I can focus on the match instead of winning or defending a title."
The snooker woman who made history for Hong Kong now heads to Britain for a tournament and, as usual, doesn't have a target for the competition.
But she does have one thing planned for 2012.
"I will run the 10km race again, and this time, I will run with more teammates!" On-yee says.