Maybe the river as it runs all the time , and because it is not a human being so it never get tired. Also maybe the wheels of the car that runs all the day and never get tired.
I guess the answer is time, i goes on and on without getting tired, river also runs all the time without getting tired too, but in the case of humans, we get tired at a certain distance of a race.
The answer to the riddle "What runs but never gets tired?" is a river. A river continuously flows, but it doesn't experience fatigue like living beings do.