Using millis() and if statement to run ultrasonic sensor and servo motors

#include "SR04.h"
#include <Servo.h>
#define TRIG_PIN 2
#define ECHO_PIN 3
SR04 sr04 = SR04(ECHO_PIN,TRIG_PIN);
long a;
long distance;
long duration;

unsigned long prevTime = millis();
Servo myservo1; //Left leg
Servo myservo2; //Right leg
int pos = 0;

void setup() {
   Serial.begin(9600);
   delay(1000);

   myservo1.attach(7);
   myservo2.attach(8);

     pinMode(TRIG_PIN,OUTPUT);
  pinMode(ECHO_PIN,INPUT);


}

void loop() {
   a=sr04.Distance();
   Serial.print(a);
   Serial.println("cm");
      digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  duration = pulseIn(ECHO_PIN, HIGH);
  distance = duration*0.034/2;

   unsigned long currentTime = millis();

    for (pos = 0; pos <= 180; pos += 1) { // goes from 0 degrees to 180 degrees
    // in steps of 1 degree
    myservo1.write(pos);              // tell servo to go to position in variable 'pos'
    
    }

     for (pos = 0; pos <= 180; pos += 1) { // goes from 0 degrees to 180 degrees
    // in steps of 1 degree
    myservo2.write(pos);              // tell servo to go to position in variable 'pos'
    
     }
     for (pos = 180; pos >= 0; pos -= 1) { // goes from 180 degrees to 0 degrees
    myservo1.write(pos);              // tell servo to go to position in variable 'pos'
                           
  }
  for (pos = 180; pos >= 0; pos -= 1) { // goes from 180 degrees to 0 degrees
    myservo2.write(pos);              // tell servo to go to position in variable 'pos'
  if(currentTime - prevTime > 1000) {
    
                             
  }
  }
}