The Trainee Technician

There was a 120 core underground cable between two telephone exchanges which are 10km apart.

Unfortunately after the cable was  laid it was found out that the cable was wrong. The thing was that all strand of wires  were not labeled. They were all of equal size and color, so that you couldn’t distinguish them.  

You are an apprentice technician and your boss  asks you to identify and label the wires at both ends without digging the cable up. You don’t have any means of transport to move from one end another. You have only one battery, a light bulb to test continuity, an adhesive tape and a pen for labeling the wires.

What is the shortest distance in kilometers you have to cover to identify and label each wire properly and correctly?