There is a social 20 km bicycle event at the Nakdong river (near Hadan station) on Saturday 13 August. Only the first 500 participants are allowed. It costs 10 000 won. You can register by filling out the google form on the event page and transferring the money to the organiser's bank account.
The google form is in Korean, but I've translated it and it's really simple to fill out. See the pictures attached for guidance.
The organisers did not share a route map, but here's a probable route map:
I've registered and hope to see some of you there!
Any information about cycling in South Korea.