Did you try pulling up on the brake pedal? That happened to me once, not quite to the same degree as I never made it to a highway. The rear brakes were hanging up in the "on" position. I don't know what I did to cause the situation, but pulling up on the brake pedal was suggested in the FSM's brake section trouble shooting guide. They have a list of symptoms and possible cures/causes listed on the last page in the brake section.
The only thing I don't recall was the pedal behaving like you describe, rock hard like as in a booster failure. My symptoms were that I could feel a 'drag' on the car at 30 mph, and I smelt the rear brakes overheating after about a block.
Tom