Why would the transmiter need more than 50 ms for a 50 cm distance?

I've never done any ultrasonic stuff, but this is something you could consider:

1) Start sending your signal
2) Wait until you actually receive your echo (or time-out)
3) If you receive an echo, switch of your transmitter and start a timer.
4) stop your timer if you no longer receive an echo.
5) read your timer.

I don't know if the above will produce a better result, or will even produce any result at all.
You will also have to consider the fact that if you switch off your transducer, it will keep on resonating for a little while, still producing a signal after switch-off

Another way to go would be to use a fixed pulse duration. Use a 555 timer (or whatever else that would do the trick) to send out a short pulse. Start measuring your echo as soon as you started your pulse. Drawback here is that you will "hear" the original signal AND the reflection. You will have to figure out what is your echo and what is not..

regards

Patrick