Oze National Park should be your next Japan hiking destination, if you haven't already been.
Oze is a marshland fresh with hiking both on the ground and up the mountains. It's located in three different prefectures: Gunma, Fukushima and Tochigi. There are two wooden walking paths in this area, totally about 9KM worth of hiking. It's an easy hike through tons of untouched lands.
I visited the area near Mount Shibutsu in Gunma. In late July, mist was covering the whole area, which gave an even more beautiful effect to the lush scenery.
Find out more about Oze National Park.