Depends how the game works, is it an infinite runner? Does the player remain at the same co-ordinates so you have faked the running or is it a platformer and he is just running to the right? You could wait a random amount of time so every random(x,y) seconds if the character is constantly running, or you can choose co-ordinates in a random range spawn at x=random(x,y) and y= always the same floor co-ordinate.