The circuit will work, but you need two measure points to process the signal.
To measure a "time-of-flight" for your ultrasonic signal, you need to have a signal that represents the time elapsed from the signal was transmitted to the moment you recieve it again.
Simply connect the output of your transmitter flip-flop V1/1 to an external counter function, and use this to start the count, and then the output of your reciever ends the count.
Divide the result by two, and you should end up with a time, representing the distance of the nearest reflection point.