No matter what, it won't be perfect. No, this is not meant to be pessimistic or anything - in fact just the opposite! Know that what you bring probably won't be perfect, and that you can
1) Donate what you don't want
2) Buy what you do
3) Mail home or mail to Santiago things that you might want back.
Shoes are the most important thing. If you live near a bigger city, find a store that specializes in fitting shoes, read Yelp reviews and pay extra for the service you get. It is worth every cent.
Socks follow shortly. Spend $30 a pair on Merino wool socks. Buy three pairs. Get geeky about socks.
Backpack immediately after.
Think carefully about cotton clothing. It is harder to dry when hand washing. But it isn't the end of the world either.
The less you carry, the happier you are.
Be prepared for blisters (you can buy blister care in Spain, but if you get a hot spot, stop and treat immediately, don't wait).
Carry something for journal/drawing. Don't rely on your camera - pictures are quick to take and this is a slow journey. Drawings and writing will encourage you to look more.
The better physical condition you are, and the slower you go, the more forgiving you can be with your gear.
Three bags inside your backpack make life so much easier, and let you use a large backpack if you already own it:
1) The clothing bag. I used a 60L backpack, cinched up as tight. Inside I had a cheap packing cube (a light weight one) with all my clothes. Made everything easier. So much easier. Best accidental decision I made.
2) The bathroom bag. a small bag that can be slung over a hook, put on the ground, on a counter or whatever that just has the things you need for your shower and morning routine - soap, shampoo, tooth paste/tooth brush, daily meds.
3) The miscellaneous other bag. I had a few small loose things that I didn't need everyday, but did want to carry along the way.
Things I bought along the way:
1) A cheap wood staff, for helping navigate the mud pits, pushing myself up hills, and supporting a grumpy ankle coming down.
2) Replacement shampoo
3) A massage ball for my foot and leg
4) Replacement soap and a container for said soap. Twice.
5) A hat. Twice.
6) A t shirt for runners. It was synthetic and dried a heck of a lot faster.
7) Ibuprofen cream
8) thingies to put between my toes (gel spacers). Wish I'd done this earlier/ wish I'd gone to a podiatrist when I first had the idea.
Things I lost, mailed or donated along the way:
- Special fancy travelers dry soap leaves for shampoo, clothes washing. Good idea. Wasn't enough soap for me in one leaf.
- My DSLR camera, e-reader, pencil crayons, cords and assorted other odds and sods. Angry feet demanded sacrifice.
- Special fancy waterproof bag I'd planned on using to wash my clothes. Just didn't work. The old school, use your hands and a bar of soap was fine day to day.
- The wood staff I bought. (forgotten)
- The first hat I bought (left behind convinced it would never be sunny again)
- Soap (forgotten)
- Ibuprofen cream (didn't work for me)