Here are some maps in English that I found for hiking near Koya-san:
http://www.shukubo.jp/eng/images/07_guide/map_1.pdf
http://www.shukubo.jp/eng/images/07_guide/map_2.pdf
Those are perhaps a bit shorter hikes than you wanted. For a longer hike, you want the Kumano Kodo:
Kumano Kodo
Kumano Kodo Maps
Maps & Pamphlets