Some 280 runners will take turns to pound the treadmill for 24 hours in the name of charity.
For every 30 kilometres clocked by each team of eight, one elderly needy patient will get free eye cataract surgery.
The event is happening at Ngee Ann City and will last till 2pm on Sunday.
The runners went through several rounds to make it to the final challenge.
But members of the public who wish to do their part can also sign up for free and run on the spot.
There will also be a booth for the needy elderly to register for the free surgery.
The organiser of the event, Mount Elizabeth Hospital, aims to have the surgeries performed by the end of the year.