start with fresh tires, carry only one spare.

I run Panaracer Minits, which are light weight, and not so easy to find at the LBS. They wear well, are very fast, and have quite good traction. I less worried about wearing the tires out than I am about damage from road debris.

unless folding bikes special consideration...

When the bike is folded, the cables are folded as well. Normally this isn't a problem, but from time to time a cable tends to pop over where it shouldn't be, and get pinched.

i'd use a small handlebar bag instead of a backpack,

I've been commuting with this backpack for a few years now. I keep it light, no need to put more weight on the saddle than I already am. The R&M bar and stem is a little top-heavy due to the design, the front suspension has a lot of rake, and even with nothing on the bar, it tends to fall to the left or right when I remove my hands.

That's a long way to go in a month in Japan, which you probably know!

Yeah, it's along ride, but I have been living in Japan for 10 years now, and have seen much of the country already. Seeing it from two wheels will be a pleasure, and if I miss something along the way, I can come back another day.

I'm guessing you already know about Mapples, or are you doing the Length of Japan Route?
At least with the folder and a rinko you can easily jump on the train...

Doing the length of Japan, it's something I have always wanted to do after reading an article in an old Bicycling magazine when I was in junior high school. Folders are very convenient in Japan, and if I overdo it, or get too far behind schedule, I am not averse to hopping on a train or bus.

For those who only have a week to cycle, that's a little rough, but for a month on the road it's worth it.

I love riding, and I've been doing it for a long time. I competed in some of the stage races in Southern Europe in the late 90's, and know about getting acclimated over a period of long rides. The bike messengers who race by me when I am shuttling my 2-year-old around on my wife's shopping bike wouldn't have been a match for me in my racing days (a fairly common boast among former racehorses, I am sure).

If you pass me along the way be sure to give me a wave, or at least let me draft.

I'll keep my eyes open for you. I'll probably be the only guy on a Mad-Max type folder. When practicing "stealth camping", keep the local wildlife in mind. Most of the hunters up north have died of old age, and the lack of hunting has led to a boom in dear, bears, and wild boars. There have been a few reports of attacks in the news.