Where is 42 kilometers of Jingzhou Highway in Fangshan District, Beijing?

I don't know the exact location. It's probably Fangshan Old Town.

42 kilometers is the length of 42 kilometers from the starting point, and the old Jingzhou Road should start from Liuliqiao. I don't remember exactly,

That's right. You can go to Jingzhou Road. Every 1 km, there will be a kilometer sign on the roadside, and you can see the distance from you now, which is 42 kilometers.