I'm going to second the notion of rolling it instead of carrying it. Most jogging or all terrain strolles are rated for 40-45 lbs per kid and 10-20 lbs of cargo. My double stroller is rated for 45 lbs per kid and 20(?) lbs of cargo. It weighs 36 lbs. ATM, it weighs 90-100 lbs when loaded for a daytrip. I can cruise on flat ground with it. The only time I really feel the weight is going uphill, and my exertion level is considerable lower then carrying half that weight.

Conversly, carrying 50 lbs on concrete is going to kill your knees and hips.

In addition to a folding game cart, which is probably the most off road capable, there's a couple of other option:
1) Folding luggage cart: Drop the backpack on it and wheel it around. Strap it to the pack when the going gets rough.
2) Trolley cart: If little old ladies can push around a weeks worth of groceries in these, you should have no problems with an oversized BOB.
3) Garden cart: My neighbor tows his two grade schoolers around the neighborhood in one of these. Not compact, but can haul an obscene amount of weight.
4) Dedicated rolling luggage: Try to find something with inline skate or scooter wheels that can be replaced in the field and carry spares.