Romans carrying 17 days rations

[Nathan Ross]

knowing what I used to carry when on infantry training!

This UK Government paper provides some stats on the loads carried by soldiers in the modern era, which might be useful in establishing the boundaries of the possible:

The Soldier's Load: Historical Data.

Some of the burdens are quite amazing; the brigade carrying 95lbs in the tropics, for example, or the poor guy in the Falklands lugging 146lbs

But training seems the important thing. No modern troops are trained specifically to march long distances under heavy loads like the Roman army were.