You can easily calculate the distance between 2 points or to objects with the system expression "distance".
From the manual :
distance(x1, y1, x2, y2) Calculate distance between to points
So at the end of the run, just get the distance between the player and the start : distance(start.x, start.y, player.x, player.y).
Edit : This can work if you are moving the start point or the player.
Else, you can increment a variable called for example "distanceTravelled" every tick.
Every tick -> add (dt * variable) to distanceTravelled