Cat5 cabling length limit for 10-BaseT networks is 100 meters.
(Even at this length, the "lag time" associated with a signal travelling over that cable is insignificant (nanoseconds?)...)
So, sending a "packet" from California to New York would not be possible using a single 10-BaseT network. You'd need to string several larger networks together, and use them to collectively "relay" the packet from California to New York.
So to answer your question, it's the "relaying" of the signal between the larger networks that increases ping, not the cable length of your local network.