I consider myself fortunate that I do not have to ride my bicycle on any roads where the speed limit is above 40. I plan my routes and commutes to purposefully stay away from those types of situations. There are plenty of jerk motorists on surface streets where you rarely see large trucks travelling at high speeds. If you absolutely don't have to, it's better not to push those limits. You will find a lot (and that is not over use of the words) of people that truly believe that the roads are only for motor vehicles. I've even been screamed at to "get off the road" when I'm in a clearly marked bike only lane.
If I were in the OP's situation, I would have probably gotten completely off the road if he were blaring his horn at be from that far back. I would have got his plate number, called it into the police, and told them he ran me off the road. The only reason for a driver to warn you from that far back is to alert you that something is really wrong (and he should have his emergency flashers on and be flashing his headlights)... i.e. his brakes are gone (but that would only apply on a downhill stretch - for anything else, it seems reasonable that a driver could shut down the engine.)