Moving a Rigidbody2D a set distance over a set time

I am trying to implement a dash move that will take a definable time to dash the rigidbody/player over a certain distance. I can get the player to dash but am trying to slow down the movement so it isn't basically instantaneous. I thought I would be able to set a smaller 'dashForce' and repeat it with a while loop until the 'dashTime' counter reaches 0 but after what seems like the first dash the player stops and doesn't continue for the entirety of 'dashTime'. I think it has something to do with the fact the FixedUpdate() updates more often then Update() but not quite sure. What am I doing wrong? Or what can I do to fix my code?

Code (CSharp):

1. private void DoDash()
2.     {
3.         if (canDash && wantToDash) // if canDash and player presses C
4.         {
5.             dashTime = startDashTime;
6.             controller.ZeroVelocity(); //Sets current velocity on player to zero
7.             controller.UseGravity(false); //Disables Gravity on Rigidbody2D
8.             while (dashTime > 0)
9.             {
10.                 if (facingRight) //Dashes in the direction the player is facing
11.                 {
12.                     Debug.Log("Dashing Right!");
13.                     controller.rb.velocity = Vector2.right * dashForce;
14.                 }
15.                 else if (!facingRight)
16.                 {
17.                     Debug.Log("Dashing Left!");
18.                     controller.rb.velocity = Vector2.left * dashForce;
19.                 }
20.                 dashTime -= Time.fixedDeltaTime;
21.             }
22.             controller.UseGravity(true); //Enables Gravity after dash ends
23.             dashCD = maxDashCD; //Resets the dash cooldown
24.         }
25.     }
26.  
27. void FixedUpdate()
28.     {
29.         DoDash();
30.     }