PING sensor and DC motor Control

So, after giving up for awhile I'm having another go at Arduino.
I am a beginner using the arduino motor shield.
My Problem:
I want to have a PING sensor on Pin 7 measure distance values, then set up a simple if/else statement to tell the DC motor to run if reading a value over 12 inches. Please be patient with me.

I have a parallax PING sensor and have tested the general PING code. I want to add in something like:

if (inches > 12) , I want the motor to go, so digitalWrite (DIR_A, HIGH) and digitalWrite (BRAKE_A, LOW) and the motor speed controlled by analogWrite (PWM_A, 255)

else (inches < 12), I want the motor to brake, so digitalWrite (DIR_A, LOW) and digitalWrite (BRAKE_A, HIGH)

However, I do not know where or exactly how to add this.

const int
pingPin = 7,
PWM_A = 3,
DIR_A = 12,
BRAKE_A = 9;


void setup()
{
  pinMode(BRAKE_A, OUTPUT);
  pinMode(DIR_A, OUTPUT);

  Serial.begin(9600);
}

void loop () {
  

  // establish variables for duration of the ping, 
  // and the distance result in inches and centimeters:
  long duration, inches, cm;

  // The PING))) is triggered by a HIGH pulse of 2 or more microseconds.
  // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
  pinMode(pingPin, OUTPUT);
  digitalWrite(pingPin, LOW);
  delayMicroseconds(2);
  digitalWrite(pingPin, HIGH);
  delayMicroseconds(5);
  digitalWrite(pingPin, LOW);

  // The same pin is used to read the signal from the PING))): a HIGH
  // pulse whose duration is the time (in microseconds) from the sending
  // of the ping to the reception of its echo off of an object.
  pinMode(pingPin, INPUT);
  duration = pulseIn(pingPin, HIGH);

  // convert the time into a distance
  inches = microsecondsToInches(duration);
  cm = microsecondsToCentimeters(duration);
  
  
  Serial.print(inches);
  Serial.print("in, ");
  Serial.print(cm);
  Serial.print("cm");
  Serial.println();
  
  
  
  delay(10);
}

long microsecondsToInches(long microseconds)
{
  
  return microseconds / 74 / 2;
}

long microsecondsToCentimeters(long microseconds)
{
 
  return microseconds / 29 / 2;

}

However, I do not know where or exactly how to add this.

After you have calculated the distance in inches would seem to be the logical place. As to how to write it, you have practically done it in your description. Look at the Reference section to see exactly how to construct if/else statements.

If you don't care about the distance in centimeters, why are you calculating the distance in centimeters?