Having worked for a Very large Fiber Optic Provider. Yes it does come in 80Km and more lengths.. to use USA measurements, the spools to be installed for the contract are hooked up together and yes it goes thru the WHOLE thing, to test all the fiber and find out if any is bad, so all that would happen in deployment would happen in the factory even when testing 3500MILES!! of single mode cable running 256 channels at 40+gig/sec per channel with NO repeaters. This has been done and are installed in the world now so this is old news that it can be done.
Now this might be a better way or new to the world, but going across the pond is old hat for some companies that use NO repeaters, I know this was being done 8+year ago, as I was on the team that tested it and help deploy it.
Note for Fiber its not like copper wire , it doesn't radiate EMF like a coil or wire does when voltage/current flows. the problem is noise and attenuation of the light itself from impurities. Better cable now and better head end electronics have made repeaters a non issue. so even in lab/factory if you put one station next to another and run it thru all the fiber its just like being out in the installation.