I'm tired and hungry after a workout, so let me just write a quick reply to acknowledge all this help--

Yes, I intend for the 40 lbs to be the theoretical upper limit. I would want to lug significantly less than that if I want to get anywhere. I just weighed my current kit (minus food & water, extra clothing, arms), and it comes out to around 18 lbs.

I have a Kelty ultralight backpack from REI. I'd mainly like to have more compartments, especially in the front, but as someone points out, that usually means more weight.

Thanks for your input, and I'll come back again when I have more questions to ask!