From 2005:
"The average distance covered in the games across all positions was 12.5km ±1.7km. This
is in contradiction to previous reports that have stated running distances of some players
were found to be upwards of 20km a game. Although these large distances may have
occurred in the past, they would appear to be atypical of a modern AFL game. The
shorter distances recorded (12-13km) may be due to the higher rotational strategy applied
to ensure that a high work rate is maintained during actual playing time. This indicates
that while the absolute distance covered in modern day AFL may be lower than
previously assessed, the intensity during on field playing periods may in actual fact be
increasing as the game, and capacity of players develops."
"One of the striking outcomes from this investigation was the similarity across the three
positions for many general work variables. Previously it had been considered that
midfielders cover the greatest distance during games, however these results confirm that
in the modern game where players are required to have greater utility, the distance
covered is similar regardless of position. The average speed, which is also similar across
positions, was lower than previously described by other researchers. However, when the
data is scrutinised more closely it is the nature of the work that midfielders undertake and
the greater exertion required that separates the midfield demands from other positions.
Despite the probable higher fitness levels of most midfielders, it is likely that midfielders
will need more frequent substitutions than forwards and defenders in order to ensure
game intensity remains high. This requirement is confirmed as this is a tactic used by
AFL clubs at present in order to maintain the high work rate of midfielder"
http://sport.fitsense.com.au/downloads/AFL_GPS_Research_Report_2005.pdf