Sculpting onto Height Map

Hey,

So I'm getting back into Blender and I have this amazing view outside where I work that I want to recreate.

I found this tutorial on pulling height maps from terrain.party, I located my location on the website and pulled in the height map onto a plane with a subsurface modifier and a displacement modifier. 

This gave me a great start on achieving accuracy but now I want to go in and sculpt onto that to add more detail. I see the terrain in "Object mode" but when I switch over to "Edit mode", it's just a flat plane again. Hooooow do I go about doing this???

Thanks in advance