On my trip to Spain from the west coast of the US, I take everything except my pocket knife and trekking poles as a carry-on in my 35L Osprey. Those items not carried on are packed in something like this,
http://www.amazon.com/dp/B005J9BCZO/?tag=casaivar02-20 To protect my sticks I pack the little bag with the multiple Amazon inflated packing pouches I will invariably collect while preparing for my walk. I would rather trash some old bag, but I have none left, so this is about as cheap as I have found for a one-time use option.
When I arrive at the airport in Madrid I take the stuff out of the temporary pack and discard the pack in a way that I hope somebody might actually make use of it.
I walk the walk, and then, in Santiago, I choose to check everything, except my most valuable electronics, with my backpack in the luggage hold. I carry on, on my return flight, something like this,
http://www.amazon.com/dp/B00IU60PCE/?tag=casaivar02-20 which is also VERY HANDY as a day to day walkaround pack for after you have you finished your daily grind. It holds your clothes, valuables, and toiletries in the shower as well as your valuables as you walk about town.
I figure I can react if my main luggage is lost on the way back, but it would suck to have that luggage gone on the day I arrive.