How to drive motors and use an ultrasonic sensor simultaneously?

I have a L298 motor driver, which I coded with an Arduino Nano. The code is very simple at the moment:

// connect motor controller pins to Arduino digital pins
// motor one
int enA = 10;
int in1 = 9;
int in2 = 8;
// motor two
int enB = 5;
int in3 = 7;
int in4 = 6;
void setup()
{
  // set all the motor control pins to outputs
  pinMode(enA, OUTPUT);
  pinMode(enB, OUTPUT);
  pinMode(in1, OUTPUT);
  pinMode(in2, OUTPUT);
  pinMode(in3, OUTPUT);
  pinMode(in4, OUTPUT);
}

void demoTwo()
{
  // this function will run the motors across the range of possible speeds
  // note that maximum speed is determined by the motor itself and the operating voltage
  // the PWM values sent by analogWrite() are fractions of the maximum speed possible 
  // by your hardware
  // turn on motors
  Motors_FWD_PWM_Initialize();

  // accelerate from zero to maximum speed
  for (int i = 0; i < 256; i++)
  {
    analogWrite(enA, i);
    analogWrite(enB, i);
    delay(20);
  } 
  // decelerate from maximum speed to zero
  for (int i = 255; i >= 0; --i)
  {
    analogWrite(enA, i);
    analogWrite(enB, i);
    delay(20);
  } 
  // now turn off motors
  MotorsOff();
}
void loop()
{
  demoTwo();
  delay(1000);
}

void Motors_FWD_PWM_Initialize()
{
  digitalWrite(in1, HIGH);
  digitalWrite(in2, LOW);  
  digitalWrite(in3, HIGH);
  digitalWrite(in4, LOW);  
}

void MotorsOff()
{
  digitalWrite(in1, LOW);
  digitalWrite(in2, LOW);  
  digitalWrite(in3, LOW);
  digitalWrite(in4, LOW);  
}

Currently I have two questions:

Q1: I have an ultrasonic sensor, how can I write the code so that the motors work and 'keep an eye out' with the ultrasonic sensor simultaneously? i.e. When a certain proximity is reached I would like the robot to stop.

Q2: Also I would like my robot to 'rove' around. How can I make it so? I was thinking having a number of subroutines, and making the arduino randomly choose one.