I used this very early on and it has not caused any hiccups.
if (Input.GetKey(KeyCode.LeftShift))
{ GetComponent<NavMeshAgent>().speed = 3.4f; }
else { GetComponent<NavMeshAgent>().speed = 1.8f; }
FUNCTION : When player holds the left shift key, when clicking on terrain or enemy the player will accelerate to a defined speed. If the left shift key is released then the player’s speed returns to its initial value;
Stick those few lines (or a facsimile that you modify) at the beginning of PlayerController.cs / Void Update() just after checking if the player is dead but BEFORE calling the mover and fighter functions.
It is a VERY basic feature but it gives me the walk or run option plus can be expanded into its own method later for polishes down the road.