Even though its winter , its a great time to start thinking about your Castle to Castle brevet kit such as - tent , sleeping bag, air mattress, cooker etc. There are lots of great outdoor shops in Japan and a wide variety of products to suit everybody's budget. When you go shopping , do some research and remember you have to carry it - where will you put it on your bike and will it fit in your seatpack, handle bar bag etc. Once the weather warms up get out on an overnight trip and learn how to use the items you purchased . Even before then you can test other items of kit out at home such as cooking some lunch on your new stove or sleeping the night on an air mattress in a sleeping bag. Everybody has a different kit list that pack for brevets so do not copy them but find out what works best for you.