Unity C# Two keys pressing one after another not doing a function vice versa

I have made this movement code for my player (Excerpt):

     if(Input.GetKey("w") && Input.GetKey(KeyCode.LeftShift))
     {
         transform.Translate((Vector3.forward) * runSpeed * Time.deltaTime);
         anim.SetFloat("Speed", runSpeed);

     }else if (Input.GetKeyUp(KeyCode.LeftShift))
     {
         transform.Translate((Vector3.forward) * walkSpeed * Time.deltaTime);
         anim.SetFloat("Speed", walkSpeed);
     }

I have tested this, and it works when I hit shift, and then "w" ( Or both at the same time), but when I hit "w", and then "shift", my character stays in it's walking animation, but speeds up. How can I make it so that when I am walking (Pressing "w"), then press "shift", my character plays the running animation, and speeds up?