I actually just got back from Japan and was going to do a write up of my trip and post some photos when i get the time. It was more a skiing trip but I took a few day walks from Osaka while i was there too.
There are literally hundreds of good walks to do, just depends on where you are based and how much you want to do. There is a big trekking (as they call it) culture there and it's not hard to find a good walk only an hour's tain ride from any major city.
The topography of Japan means most of the mountains are untouched and provide plenty of good walks. Most of the guidebooks are based around going to one peak or another. There is a challenge to climb all of Japan's '100 famous mountains' and for the most part, do not require any climbing gear.
From my experience the best and easiest to get to are the ranges of central Honshu. If you're not familiar with the place, around Nagano and Niigata there are literally hundreds of walks on well-defined trails.
If you're into gear, the stores are amazing too. The Mont Bell mothership in Osaka was like a supermarket of outdoor gear (I ended up spending a small fortune and about 3 hours wandering about the place). Or try another one called 'Kojitsusanso' (
www.kojitusanso.jp) which has everything you could possible need plus plenty of gadgets you won't! There are also about 4-5 monthly magazines devoted to trekking and I even found a weekly one-hour TV show which featured a walk and gear reviews. This store also runs guided tours of various durations all through the year, with different levels of difficulty and length.
I've been many times, but rarely outside winter so haven't even scraped the surface of the walks available. To me, it's one of those places you don't usually consider for bushwalking but has a lot to offer. Witht he yen on the fast road to collapse, it's getting cheaper and cheaper all the time too!
I'll post some more about my trip later on.