Remember that the default FSX C172, in a cold & dark (avionics ON) setting, is my default flight. All I do is this:

1. Start FSX.
2. In the Start Screen, select whatever aircraft I want to fly, select whatever weather I want (but I usually use AS2012 for real world weather) and select any airport I want to start a flight from.
3. Press FLY NOW.

Of course any of my advanced addon aircraft (including the CS757) will be in a cold and dark state, so I have to setup the payload, fuel and the FMC. I then start the aircraft, taxi to the departure runway, take off and fly the route. The 757 flies the route from beginning to end usually without any problems.

If I have no TD in the flight plan, I sort it out, if I can, before departing the gate.