Check out example codes for "C# sprint key". It will help you in understanding the concepts better.

Code Example 1

if (Input.GetKey("left shift")){ // If Player Presses Shift Key
	Sprint = 2f; // Sprint Double Speed
} else {
	Sprint = 1f; // Disable Sprint
}

// Move Player
controller.Move(move * speed * Sprint * Time.deltaTime); 
// Move is your Player Move Variable

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