This is an open event by Outdoor Club Japan. Wilderness spot in Yamanashi near Shosenkyo gorge and top 100 mountains Mizugaki and Kinpu San. Most of the hiking is easy and will be something every day. 3 days but 2 possible staying on comfortable lodge (or your tent- free). Heated lodges with futons and everything provided. Cost shared between people (approx 2000yen/night if 4-5 people in a lodge). Turkey, hot spiced wine, and member contributions. Food cost is also shared. Campfire each night. Most people travelling there by hire car which is cheaper than public transport. All details are on the club website: http://outdoorclubjapan.com/viewtopic.php?f=39&t=3751
To sign up you need to register – membership is free. For each event there is a 200yen contribution for website costs. This is a great trip and often repeated in December.