It's been awhile since I've done the longer hikes in Seoraksan, and I generally plan to stay in a shelter, but it looks like your course is a long
day! In normal conditions, the times on the maps are pretty accurate. As I am in the US now, I don't have a Seorak map handy. There are several on the web
, though I didn't find exactly what you're looking for (I didn't look too long^^).
Weather can be a problem, so you need to prepare accordingly. If you needed, however, there are several shelters along the way you could take refuge!