Answer: Seongpanak Trail + Gwaneumsa Trail
The Seongpanak Trail is the best Hallasan hiking trail to go up for most hikers. It’s the longest Hallasan hiking trail at 9.7km, but it starts gradual and is a walk in the park (plus some stairs) for the majority of it. The steepest part of the Seongpanak trail is near the summit, where you just have to take flights and flights of wooden stairs. Easy peasy.
The best Hallasan hiking trail to take down from the summit is the Gwaneumsa Trail. The Gwaneumsa Trail connects to Seongpanak at Hallasan’s summit, so it’s easy to navigate hiking up Seongpanak and down Gwaneumsa Trail. This trail is 8.7km long, for a combined 18.4 km day of hiking Hallasan!
For me to hike Hallasan up the Seongpanak Trail took 3 hours, and and down via the Gwaneumsa Trail took 2.5 hours. Expect this exact Hallasan hike to take about 6-8 hours, including time for resting at the summit.