Have walked parts of the Camino Frances 2 times and the Portuguese Camino from Porto. Will be 65 this summer and carry my own back-pack which weighs 6.5kg without water. Heading back to Burgos in Oct. for my next Camino.
Yes, check out the Camino forum that was already mentioned. It is invaluable. As to shoes, boots, or sandals, that is personal. Ponchos or raincoats get the same personal opinions.
My poncho has an extra pocket on the back that fits over my back-pack nicely so the poncho doesn't blow around and everything stays dry, even in the pouring rain and high winds. Meindl boots are my choice and I wear nylon knee highs with wool socks (both turned inside out if they have seams) and have yet to get a blister.
Every Albergue and hotel has envelopes for you to use to transfer luggage. This isn't something you need to plan ahead too much. Many albergues also have private rooms so that you can still be with other pilgrims at meals, etc.
Recommend booking your bed in Orisson and Roncesvalles as soon as possible.
Personally, I like to keep my daily walk to about 20km. This allows me to get to the albergues in time to shower, wash my clothes, have a bit of lunch, etc. Dinner is usually at 19:00 or 20:00, get my pack ready for the next day with my clean laundry, and then collapse in bed at 21:00. Up early, get coffee or breakfast and head out to catch the sunrise.
In Santiago, plan a few days at least. Go on the bus tour to Muxxia and Finesterra. Tour the cathedral. Train back to Madrid or bus to Porto, whichever is best for you to get home.