That is an interesting question, somewhat out of my area of expertise of animal sciences, although I have built about a half mile of electric fence on our own place.  Typical anti-predator electric fences have alternating hot and ground wires and use ground rods along the fence to keep the grounding consistent along the fence.    The in line ground rods can be hundreds of feet away from the energizer.

So while I can't give you a solid answer, it would seem to me that a distance of a 100 feet would not cause an issue.  I am also not sure you would need an insulated wire to connect to the ground rods as a regular galvanized wire might add to the grounding but I am not sure on that point.  Long time fencing companies like Pel and Gallagher used to have technical staff to help with questions but I am not sure those exist today.  You could look on their web sites or use the company that made your charger if different.

If you need more information my phone number is<personal data hidden>

The energizer manufacturers used to print books on building fences and also had a protocol to use to check the ground capability of the fence.  Maybe that information exists on a web site and you could use it to check the efficiency of your grounding set up.
A fence volt meter is a useful tool to work on electric fences.  I like the digital ones that list the actual volts.  Can be very useful to locate voltage drops.