Hi Christine. To answer some of your questions:
A quick internet check brought up a lot of photos of the trail, and it doesn't seem to be a "wilderness" trail at all, but consisting of a lot of road walks. How much road walk is there really involved? Any idea of the percentage?
The Tokai Shizen Hodo is not a wilderness trail, though some long sections of it are in the mountains, especially in Tanzawa (just east of Mt. Fuji), the southern foothills of the South Alps (just west of Mt. Fuji. Most of the trail stays on hiking trails and not on roads, though there are road sections. It is usually not far from roads and towns. Japan has few real "wilderness" areas, unless you go to the deep mountains, where slopes are so steep in most places that no one can walk there. I would say that hiking in the mountains in Japan is very similar to hiking in the Alps; there are just too many people in Japan to offer the huge, unpopulated expanses of America. In fact, I was surprised when I went to the Alps last year by how similar the trails and huts were. I would say that the mountains in Japan are wilder than those of Europe, though. And much less regulated.
Also: Where would you camp on such a trail? Japan is about the safest country in the world (one of the reasons why I chose to go there), but how do you manage to camp there? I attended a Japanese language course in preparation for this trip and my teacher advised me on asking in temples for a place to camp. What are you doing?
There are quite a few camping areas in Japan, especially at lower altitudes. All along the trail you will find campgrounds with baths and showers. Also, the Japanese are very tolerant of unusual occurrences. I have "wild-camped" ( nojuku ) throughout Japan in playgrounds, small neighborhood parks, temples, riversides, country train stations, beaches, once, even, in a public toilet, and never once had a problem. In fact, these were some of the best places to meet people throughout the country, and often I was invited to stay at people's homes and I made some great friends. Such travel has an ancient history here so people understand the desire to do it very well.
And: What maps should I buy? Is there a certain map series you can recommend? Would the trail be marked there? And how would it be marked in nature?
The "Yama to Kogen Chizu" ( 山と高原地図 ) series of maps ( 1:50,000 ) put out by Shobunsha ( http://yamachizu.net/ ) is probably the best general purpose maps out there for hiking. You can buy them at any bookstore and their maps cover all the hiking regions of the country. If you want more detailed/ accurate maps bigger bookstores will have special drawers in the map section of the store that contain 1:25,000 sized, very accurate contour maps of every section of the entire Japanese archipelago. I use these for when I am hiking off trail.
Trail blazing and signs are in general very well marked and there are sign posts and things like painted markers or strips of bright colored cloth all along all the trails everywhere. It is hard to get lost in the mountains here if you keep an eye on the map. However, do remember that in the mountains, unlike the trains, all the signs will be in Japanese, sometimes with indecipherable readings that even the Japanese often cannot read.
My internet research revealed that there should be other nature trails in other prefectures, for example an Shikoko Nature Trail. Do these other trails exist only on paper or in reality, too?
Japan has trails all over the place! LIterally thousands and thousands of them. Many of them are only locally known and no book will show them. I've never been to Shikoku so I don't know what mountains there are there, except for Ishizuchi Yama, which is supposed to be sublime. The mountains in Shikoku are very rugged and steep and so there aren't a lot of longer trails. BUt to answer your questions, there are lots of nature trails in Japan, perhaps too many to help you in one simple post.
And: Is the 88 temple pilgrimage worth considering to hike? I am very much deterred by the high percentage of road walk, but there is an English guide book about it.
Certainly it is worth it. But you have to consider that it is a religious walk, not a wilderness walk. It is similar to the Camino de Santiago, and stays along the roads and temples and towns that strung all these points together from hundreds of years ago. One thing though... there will be hordes of people! Most people no longer walk the route, but take buses... buses packed to the gills! Beware of any tourist routes anywhere in the country. Japanese love doing things en masse. (for instance stay away from Oze. You will literally be waiting on line behind hundreds of walkers as you follow the trail)
Hi Ed. Now on to your questions:
Temperatures in the hills...
In July and August? Well, expect to be drenched in sweat and constant humidity. If you go high up, say over 2,000 meters, it will be cooler, but be prepared for either lots of sunburn, or, if the weather doesn't improve this year, lots of cold nights with heavy rains. I'd say think of the Alps in Italy in August, but without the snow.
Showers/sentos in refuges...
Most of the bigger refuges in the North and South Alps have their own ofuros (sentos are public town baths), especially in the North Alps. For any walk in the Chubu region you shouldn't go for more than two days without a bath. There are also many open air hot springs here and there, too.
Well, sea slugs are not a common item on any menu, let me tell you! You should be fine. Most places offer hot meals, usually things like curry rice, ramen, or home-style dinner sets. Usually there is a choice. You can also buy food for the trail, including (limited) raw items for cooking. Beer can be found everywhere.
How to get around cheaply....
There are a number of special "hiking passes" for trains and buses, but putting together the information is a nightmare. There is little information in English. Definitely get the Japan Railpass. It will save you hundreds of Euros as you travel about on trains and even buses.
Best multi-day (hard) routes....
My favorite multiday, high mountain routes are:
Three or four day traverse of the Shirane Three Peaks, South Alps.
Four day traverse of the northern section of Hakuba, North Alps.
And my favorite walk in Japan, the five day crossing of Kurobe Goro, North Alps.
All are stunning and unforgettable.
What clothes/bag/tent/gear to take....you get the idea ;-)
I would say think the same way you think about the Alps in Europe, though not quite a cold, nor as high. And if you're going to camp on the ridges make sure you have a tent which doesn't have too large a footprint. Space is a premium in the mountain in Japan because everything is so steep.
I have to go to work. I'll write more later.