how to make the character run forward by default without player command??

i’m making 3D a maze game where the player will only have to give side and jump controls similar to an infinite runner.
but i’m not able to make the runner move forward by default.i added the code to my charater script:
transform.Translate(5f * Time.deltaTime, 0f, 0f);
but the player rather than running on the plane is jumping in the air every time it touches the ground.what should i do.

entire script

/// This script moves the character controller forward 
/// and sideways based on the arrow keys.
/// It also jumps when pressing space.
/// Make sure to attach a character controller to the same game object.
/// It is recommended that you make only one call to Move or SimpleMove per frame.	
var speed : float = 6.0;
var jumpSpeed : float = 8.0;
var gravity : float = 20.0;
private var moveDirection : Vector3 =;

function Update() {

transform.Translate(5f * Time.deltaTime, 0f, 0f);	
var controller : CharacterController = GetComponent(CharacterController);
	if (controller.isGrounded) {
		// We are grounded, so recalculate
		// move direction directly from axes
		moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,
		moveDirection = transform.TransformDirection(moveDirection);
		moveDirection *= speed;
		if (Input.GetButton ("Jump")) {
			moveDirection.y = jumpSpeed;
	// Apply gravity
	moveDirection.y -= gravity * Time.deltaTime;
	// Move the controller
	controller.Move(moveDirection * Time.deltaTime);

Just omit the vertical input. Change:

moveDirection = Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));

For this:

moveDirection = Vector3(Input.GetAxis("Horizontal"), 0, 1);