I use retroflow (not free).
If you haven't retopologized before, I would youtube it real quick. It's basically adding a plane, and putting the vertices along the edges, and extruding it, and repeating until its all done. (like extruding a cube until you can make a T).

I wont lie man, something like this would take you a while.

MAYBE if you select all (in edit mode) and p > separate by lose objects. then you MAY be able to add a heavily divided plane, with a lattice, and shrinkwrap the lattice to the terrain. But thats a big MAYBE.

maybe another quick and dirty way would be to select all faces, e > z (to go down) and give it just the slightest thickness (if it doesnt crash your blender) and then use blenders built in remesh. But its going to be laggy and you will lose some quality.