Thank you very much for all the responses.

I am pleased to hear that I should be able to pack much lighter than 20kg. I definitely don't want to take any unnecessary stuff which will only add to the weight. As for electronics, I want to take only three things: phone, power bank, charger.

I would like to discuss my potential packing list. I have already seen the link provided by BZH, so let's go through most of the items from that list:

• Backpack - after some investigation I decided to go with the same backpack. It's a Deuter Futura Vario 50+10. The spec says that it can hold up to 15-18kg and weights 2kg.
• Trail maps - the Japanese Ministry of Environment has maps in PDF form for the whole trail. I will download them and take them with me.
• Compact first aid kit - I will probably need this, although I am considering taking only painkillers, some bandage and fingerprint protection. There will always be a town close enough to get there in case of needing help, so maybe I don't need the whole kit?
• Lightweight tent, sleeping mat, sleeping bag - definitely needed.
• Headlamp - needed. I am sure the batteries won't last the whole trip, especially if I use it inside the tent. Should I take spares with me or look for them in towns as I hike along?
• Solar-charged lamp - although I could use it inside the tent, wouldn't it be enough to use the headlamp? Seems unnecessary to me.
• Portable camping stove and gas - this is a tough one. On one hand, it would be nice to have one and be able to prepare hot food. On the other hand, I don't even know if Japanese convenience stores sell anything that can be prepared this way. I also didn't investigate how much gas would be needed for such a long trip.
• Toiletries - I will need sunscreen and an insect repellent.
• Quick dry towel - yup.
• Sunglasses and hat - I don't wear sunglasses, but a hat is required.
• Light-weight rope - could be useful for hanging wet clothes to dry off. But maybe I can just hang them on a tree branch or something?
• Portable cup, metal reusable chopsticks, cutlery - I will take cutlery and maybe a cup.
• Swiss army knife - yes.
• Portable battery - I guess he means a power bank. As mentioned earlier, I will have one.
• Portable solar panel - don't need it if I am not taking a solar-charged lamp.
• Plastic bags - needed for dirty clothes and to protect stuff from rain.
• Ziplock bags - needed to waterproof the most important items.

All right, now it's time to decide what clothes I need to take:

• Shoes - I will most likely use the Merrell Track Glove 5. They are intended for barefoot usage but I have read that people are using them with socks and it works fine.
• Lower body - I think I will need trousers and shorts (I have no clue how many I actually need; my gut tells me that I need two of each - whenever one is drying off, I can wear the other one). One pair of trousers has to be waterproof. To save space, I can buy trousers which have a zipper mid-way that transforms them into shorts.
• Upper body - I think I will need t-shirts and long sleeve shirts (again, I have no clue how many). Also a light waterproof jacket.
• Socks and underwear - the only question here is: how many?

I will also need some more toiletries: soap, toothpaste & toothbrush, maybe a shampoo (although I can just use soap).

Please let me know what you think and if I missed anything. And can someone tell me how much could all the stuff that I should bring roughly weigh? Is it really possible that it could be 12kg including water and food for one day? All constructive criticism is greatly appreciated!

Radek