I know it depends on mass and speed, so take for example the asteroid that caused dinosaurs mass extinction. I read that it was 6km (10 miles) wide. How fast did the destruction - trees, rocks, dirt, debris - traveled? Was it the same speed as the shockwave?
I think you multiplied instead of dividing for km/miles. Either way, that is a big rock going very fast. Won't the atmosphere slow it down? Not really. Its diameter is as much as the thicker part of the atmosphere. There is a second or less from hitting the whips of atmosphere and hitting the surface.
Super-heated compressed air will be pushed at hypersonic (or orbital) speeds - fast enough and hot enough to cut through rock- and contact with the ground will squirt material out at those speeds as well. Molten white hot material mixed with cooler material pushed by it and it will travel at hypersonic speeds for great distances with some going into space and coming down anywhere on Earth. The atmospheric shockwave will be supersonic for a long ways then become the speed of sound. Note the speed of sound is determined by temperature, not density. Compression by the shock wave will have some strange heating effects.
All in all, a very bad day but quite something to see from a safe distance. By the way, it would looke rather slow. Hollywood explosions of this sort move faster than light if you do thumbnail calculations.