You'd die if you tried to reach those speeds on any road I've ever seen.

[A cart hitting a rock when pulled by an ox gets stuck. A cart hitting a rock pulled by a trotting horse breaks a wheel. A cart hitting a rock at... he has to stop to do math again, the unfamiliar units giving him pause. A cart hitting a rock four or five times as fast as a horse's gallop?]

But if I'm doing the math right, I think Shangri-la is a bit larger than your country, from the coastline to the provinces.