Time between two echos will be very short.
That depends on how often you send a pulse.
Send a pulse. Get an echo. Record the distance based on the time to get the echo.
Wait a bit (say 5 milliseconds).
Send a pulse. Get an echo. Record the distance based on the time to get the echo.
Wait a bit (say 5 milliseconds).
Send a pulse. Get an echo. Record the distance based on the time to get the echo.
Three pulses. Three times (or distances).
Yes, the individual times will be small. The relative time, though, will be quite a bit larger.
Your way of blindly doing nothing but pinging will have your robot dead in no time.