How fast does electricity travel through dirt? They are talking hundreds of miles in some cases with a dirt return. Just how conductive is 240 miles of dirt?
Good question, but you're looking at the concept incorrectly.
You need to consider the resistance of the ground rod to earth. The National Electric Code expects a resistance of 25 ohms or less using a single ground rod.
OK. If you assume a load of 10 KW off of a 14 KV power line, the AC current drawn is 700 mA. Not even an amp. If the ground rod return is 25 ohms to earth, the voltage drop is only about 17 volts, less than 1/10 of 1%.
As far as the 250 mile distance goes, the earth itself has negligible resistance, it's the resistance of the ground electrodes *to* the earth that counts. Heck, the center of the earth is a ball of metallic iron and nickel.
What would really stink with a 1-wire power feed is what happens in the event of a lightning strike!
250.56 Resistance of Ground Rod Electrode.When the resistance of a single ground rod is over 25 ohms, an additional electrode is required to augment the ground rod electrode, and it must be installed not less than 6 ft away. 
