I too have the E-Flite P-51 Mustang and it flies BEAUTIFULLY. If you haven't flown it yet, don't be intimidated, it handles great and is super stable, especially with the AS3X. Just follow the manual for setup and make sure everything functions correctly then just fly it like any other plane. Take off at around 1/2 throttle (NOT FULL) and I don't use any flaps for take off (you can, but I don't). Be aware that there is P-factor on it because of the design so some rudder is required on take off. It is OK if you need to abort your first take off because of the unexpected P-factor, I did. Once the tail wheel goes up just keep it straight and pull back. On landing, practice with plenty of space and be prepared to overshoot your first couple of times. Once the mains touch just bring the elevator all the way back and power off. I know what I just said sounds kind of frightening, it really isn't. Don't overthink it and use your skills and experiences. Let me know if you have any other questions!
Sky Pirate