This is the current fitness function for the running task:
fitness = Mathf.Clamp(creature.GetXPosition() / (MAX_DISTANCE * SimulationTime), 0f, 1f);
The fitness is directly proportional to the horizontal distance that the creature has travelled, the top speed has no effect.
I'm going to look into this to see if I can reproduce this the next time I have time to work on this project again.