I'd suggest to break out the hammer drill, drill a hole close to the load center you installed, and install a ground rod. Yeah, drilling through old barn floors and feed lot slabs can be tedious but a ground rod with short path to the load center is an ideal situation. If any device does short to ground, the length of the ground wire will be energized. A jacketed wire won't be too much of an issue but if the ground wire is bare copper, any flammable stuff on it may ignite.
It sounds like there was little thought to bringing any new service up to a reasonable state by the owner or past occupants. Now would be the time to get it as close to good as possible.