Please read below to give you some ideas and advice on walking in Japan
Equipment for a one day trip
This section is termed comfort as opposed to safety. When stopped, put on an extra layer of warm/windproof clothing, as you get cold very rapidly. Also don't start the walk with all your clothing on - you will get hot quickly. Carry some reserve clothing for when it gets cold.
Eating carbohydrate foods beforehand is a good idea (pasta or potatoes for example). On the mountain eat a little, but often. Chocolate bars are a good source of energy, giving nibbles to accompany sandwiches etc. Dried fruit is also full of energy. Some extra food is needed as emergency rations in case you get delayed.