Hi, Haylene!
Here are a few multi-day hikes in Jeollanam-do, South Korea:
1. Jirisan Mountain in Gurye-gun; Best time: September to October
2. Ngodan hiking trail in Jirisan National Park; Best time: September to October
3. Wolchulsan National Park hike; Best time: September to mid-November. Check out the following link to find more such hikes:
https://www.tripoto.com/hiking/in-south-korea