You will be carrying much lighter packs than I had envisioned from your first post (In North America, "backpacking" usually requires carrying all food, cooking gear, etc. self contained for the entire trip). I should have realized that most campaigns in the Ardennes have used verhicular support!

A ten kilo load should do fine in a quality day pack. Will you carry your sleeping bags and tents or are they taken ahead to the next spot as well?

I would pay particular attention to your boots, as well. Blisters and other foot gear related problems can turn any hike into misery. Walk in them lots beforehand..