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.