How to speed up my character?

I have a character in a 3D game that moves around, the problem is he is very slow .I've tried to speed him up by adding a force ,it works perfectly, but it creates another problem which is the character starts to go inside walls because of this force.. How can i speed him up using another way? This is the code:

 using UnityEngine;
 using System.Collections;
 
 public class Final_PlayerMovement : MonoBehaviour {
     GameObject ins;
     Transform mainCamera;        //holds the main camera as a Transform object
     public static Animator animController;    //holds the player character's animator controller
     public float rotationSpeed=80;
     public float speed=1000000;
     Rigidbody RG;
     //the character's possible movement states
     enum MovementState
     {
         Idle = 0,
         Walking = 1,
         Running = 2
     }
 
     //holds the player's current enumerated movement state
     MovementState currentMovementState;
     
     // Use this for initialization
     void Start () {
         mainCamera = Camera.main.transform;        //initialize the camera's transform
         animController = GetComponent<Animator>();    //initialize the animator controller
         RG=this.gameObject.GetComponent<Rigidbody>();
     }
     
     // Update is called once per frame
     void Update ()
     {
         
         //get axis input from player (W, A, S, and D keys/Arrow keys)
         float verticalAxis = Input.GetAxis("Vertical");
         float horizontalAxis = Input.GetAxis("Horizontal");
 
         //get the forward-facing direction of the camera
         Vector3 cameraForward = mainCamera.TransformDirection(Vector3.forward);
         cameraForward.y = 0;    //set to 0 because of camera rotation on the X axis
 
         //get the right-facing direction of the camera
         Vector3 cameraRight = mainCamera.TransformDirection(Vector3.right);
 
         //determine the direction the player will face based on input and the camera's right and forward directions
         Vector3 targetDirection = horizontalAxis * cameraRight + verticalAxis * cameraForward;
 
         //normalize the direction the player should face
         Vector3 lookDirection = targetDirection.normalized;
 
         //rotate the player to face the correct direction ONLY if there is any player input
         if (lookDirection != Vector3.zero)
             transform.rotation = Quaternion.LookRotation(lookDirection);
 
         //if there is any player input...
             if (verticalAxis != 0 || horizontalAxis != 0) {
 
                 //if the run modifier key is pressed
             if (Input.GetKey (KeyCode.LeftShift) || Input.GetKey (KeyCode.RightShift)) {
                 currentMovementState = MovementState.Running;    //set movement state to running
                 RG.AddForce (targetDirection * 10000000);
 
             } else {    //if there is no movement modifier
                 currentMovementState = MovementState.Walking;
                 RG.AddForce( targetDirection*5000000 );
             }//set movement state to walking
             } else {    //default to idle if no input
                 currentMovementState = MovementState.Idle;    //set movement state to idle
             }
 
 
         //send the current movement state to the character controller
         animController.SetInteger("MovementState", (int)currentMovementState);
 }
 }