"Auf Dauer läuft er nicht."
freecity: "Auf Dauer" is an expression meaning "In the long run". The rest should be simple to understand.
The problem is that it doesn't flow well in English. "In the long run" is only used as an idiom, and not when referring to long distance races.
Is it possible to translate "auf Dauer" as "on endurance" or something similar?
In the long run often refers to the ambiguous future in which events turn out to be this way or that way....
"he doesn't run in the long term" probably means that there is an irreversible damage to his body and that he could never be able to run again. Or he was considering the race for a long time but in the end he didn't as philster043 points out.
Still it's not a natural translation.
I translated this "He is not in it for the long run", which as accepted as correct.