For meeting the part 135 cross country requirement of 500 hours, check out FAR 61.1, the definition of cross country time
time acquired during a flight --
(A) Conducted by a person who holds a pilot certificate;
(B) Conducted in an aircraft;
(C) That includes a landing at a point other than the point of departure; and
(D) That involves the use of dead reckoning, pilotage, electronic navigation aids, radio aids, or other navigation systems to navigate to the landing point.
This means that on each one of your instructing flights you could head over to Riverview or Holland or any of the private fields around Z98 and do a touch and go and the whole flight is considered cross country.
The 500 hour cross country requirement for the ATP certificate is totally different than the 500 hour cross country requirement for FAR 135:
For the purpose of meeting the aeronautical experience requirements for an airline transport pilot certificate (except with a rotorcraft category rating), time acquired during a flight --
(A) Conducted in an appropriate aircraft;
(B) That is at least a straight-line distance of more than 50 nautical miles from the original point of departure; and
(C) That involves the use of dead reckoning, pilotage, electronic navigation aids, radio aids, or other navigation systems.