Making player character walk slower when further from centre

I want a basic code or if statement without writing 25 lines of code for every 10 metres or whatever. Here is what I have so far

 public float Speed;
 
 void Start ()
 {
 
 Speed = 10;
 
 }
 
 void FixedUpdate ()
 {
 
 if(transform.position.x >= 15.0f)
 {
 Speed = 5;
 }
 else if(transform.position.x <=15.0f)
 {
 Speed = 10;
 }

I don't want to add another code for y if I can save time, since my character doesn't travel in the z it doesn't matter to me.

But is there anyway to say every 10 metres or every 15 metres half the current speed?