If you want a more confortable ride from Danyang town to Sobaeksan National Park or travelling in a group of about 4 people, you can consider taking a taxi instead of the bus.
There are taxis waiting at the taxi stand just outside the town's bus terminal next to the river and big bridge, especially in the morning. The taxi cost less than US$10 per trip and the journey takes about 15 minutes through some beautiful mountain scenery.
The best and cheapest way to reach Sobaeksan National Park from Danyang town is to take the bus. The bus interval is about 1 hour and it costs less than US$1 per trip. Total time is about 30 minutes which will send you right in front of the park entrance to the national park.
Where to take the bus from Danyang? Well, you can take it at the bus stop just outside of the town's bus terminal located next to the river and a big bridge (you cannot miss this centralised area as the town is not very big).