It's pretty simple, just add the distance the player has moved every tick to a variable, then check when the variable is greater than or equal to the certain distance.
To measure the distance the player has moved you need to save the player's position to some variables. Give your player sprite 3 instance variables and call them "dist", "oldx" and "oldy". Then every tick do these three things:
1. add distance(self.x, self.y, self.oldx, self.oldy) to dist
2. set oldx to self.x
3. set oldy to self.y
You will probably want to set oldx and oldy at the start of layout to the players position, otherwise the distance from (0,0) to the player will be used.