I'm curious about this bag. How is it "firearm-centric"? If I had to be on foot for 24 hours, I wouldn't want unnecessary weight. To me that means at most a handgun with a few magazines -- and that means a handgun you can afford to lose at checkpoints, rather than your $4,000 competition gun. So it's not much different from day to day CC.

I thought about the OP's question, and it seems to me that a lot will depend on the particular situation you are anticipating. I like the general tips from hikermor and chaos, but some people don't even need a GHB at all. Maybe if we have a better idea of the scenarios you have in mind...