I've used my Osprey Atmos 65 for multi night hikes. In looking at this page (never heard of the Camino myself - but now that I'm looking at it, it reminds of a Paulo Cohello book I read a while back, can't remember the name of it though!)

http://francistapon.com/Travels/Spai...Santiago-Sucks

Doesn't look like you need to carry much of anything - honestly, the way this looks, a simple day pack will do. Carry 3 pairs of lightweight socks, 2 pairs Ex-Officio underwear, and either Vdri or Airism undershirts, and you'll be fine. Maybe 1-2 shirts in addition to what you're wearing and 1 pair of shorts or lightweight trekking pants to walk in + 1 water bottle that can be refilled easily & a couple of granola bars each day and you should be fine.

Not sure your physical condition, but 10 miles a day is basically about 6 hours of walking...you may cover more ground than you think, especially since it seems like a pretty flat walk.