The hardest part is not the cable, or even the installation thereof, it's finding a contiguous right-of-way you can install it on, or having to fuss around with thousands of land owners who want their cut for the rights.
This is why most cables are laid alongside train tracks, not unlike telegram wires of years past. You negotiate with a handful of entities and can get permission relatively easily.
Of course, this presumes there's tracks, which is less and less likely as they keep getting ripped out.
If there's anything modern infrastructure build needs, it's more right-of-way access where you can provision dark fibre without getting mired in the legal issues.
Very hard, especially when the towns are much more dispersed than a km apart.