BURNET, Tex. — I’ve driven just about everything on four wheels, but this is my first time on eight. And it’s the first time I’ve arrived at the edge of a river without stopping and turning around.

I’m strapped into a stretcher, held in by three belts. If this situation were real, it might be the only way I could be hauled out – injured – from some remote wilderness, or disaster or war zone. I’m ...