It is a long walk.

But [MENTION=12611]nerd[/MENTION] is right. More specifically, the AirTrain was designed in the 1990s and constructed from 1998-2002. Operations at the old T5 (the TWA Flight Center, now part of the hotel) continued until 2001, and operations at the old T6 (the Sundrome, now demolished) continued until 2008. So the AirTrain station (originally labeled T5/T6) was presumably designed to serve those two old terminals, both of which were closer to the station than the current T5 (which didn't break ground until 2005, and I think was originally supposed to somehow incorporate the TWA head house).