One of my main concerns about walking the Tokai Shizen Hodo nature trail was the absence of a comprehensive map that outlined the trail in fine detail. As for maps in english, they just don't exist. There are many other maps in Japanese, but none of these seemed to be comprehensive enough, only showing small sections of the trail. The best ones were user created with detailed instructions in Japanese which I cannot read very well. Whilst I have read the trail is well marked for most of the way, my concern was that if I lost the trail for whatever reason would it take me several hours or even a day to find again, and thus spend much of my time looking for the trail rather than actually walking it.
What I needed was a GPX file for my GPS that included the whole trail in detail. After several hours of searching and translating Japanese pages into English, I am glad to say that I found one that appears to be just what I was looking for. I managed to track down the webpage of a Japanese man who has done the whole trail and recorded it on his GPS. Now whilst I cannot vouch for the accuracy of the trail, it appears to correspond correctly with all other maps I have compared it to. So I think it is quite accurate. But I will have the definitive answer for you on its accuracy after I have walked it myself following this man's route.
The GPX file is in 13 parts and can be downloaded from here. You can knit the files together on Garmin's Base Camp application and presto, you have what seems to be a finely detailed trail map of the entire main section of the Tokai Shizen Hodo. One thing to note however is that it does not appear to contain the large Kansai deviation section of the trail that adds a few hundred kilometres onto the walk. Whilst I would have liked this, beggars can't be choosers, i'm happy that I found this at all.