Run speed in world-based character controller

I'm making a character controller based on world coordinates, but I can't seem to get the speed to increase when the spacebar is pressed. Here's my code:

 using UnityEngine;
 using System.Collections;
 
 public class IsoCharacterController : MonoBehaviour
 {
     public float walkSpeed;
     float walkX, walkZ, lookX, lookZ;
     public CharacterController controller; // dragged into inspector
     Vector3 lookTarget;
 
     void Update ()
     {
         if (Input.GetKey(KeyCode.Space)) // Trying to get speed to increase when space is pressed
             walkSpeed = 9f;
         else
             walkSpeed = 6f;
 
         if (Input.GetKey(KeyCode.W) && ! Input.GetKey(KeyCode.S)) // Moving up
         {
             lookZ = transform.position.z - 1f;
             if (Input.GetKey(KeyCode.D) || Input.GetKey(KeyCode.A))
             {
                 walkZ = -walkSpeed/2;
             }
             else
             {
                 walkZ = -walkSpeed;
             }
         }
 
         if (Input.GetKey(KeyCode.S) && ! Input.GetKey(KeyCode.W)) // Moving down
         {
             lookZ = transform.position.z + 1f;
             if (Input.GetKey(KeyCode.D) || Input.GetKey(KeyCode.A))
             {
                 walkZ = walkSpeed/2;
             }
             else
             {
                 walkZ = walkSpeed*20f;
             }
         }
 
         if (Input.GetKey(KeyCode.D) && ! Input.GetKey(KeyCode.A)) // Moving right
         {
             lookX = transform.position.x - 1f;
             if (Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.S))
             {
                 walkX = -walkSpeed/2;
             }
             else
             {
                 walkX = -walkSpeed;
             }
         }
 
         if (Input.GetKey(KeyCode.A) && ! Input.GetKey(KeyCode.D)) // Moving left
         {
             lookX = transform.position.x + 1f;
             if (Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.S))
             {
                 walkX = walkSpeed/2;
             }
             else
             {
                 walkX = walkSpeed;
             }
         }
 
         if (Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.A) || Input.GetKey(KeyCode.S) || Input.GetKey(KeyCode.D))
         {
             lookTarget = (new Vector3(lookX, transform.position.y, lookZ));
             transform.LookAt(lookTarget);
         }
         lookX = transform.position.x;
         lookZ = transform.position.z;
         controller.SimpleMove(new Vector3(walkX, 0, walkZ));
         walkX = 0f;
         walkZ = 0f;
     }
 }

Thanks for any help!