The main differences between flying a Cessna 172 (or most light GA aircraft) and a Boeing 757/767 are the systems and obviously the size & mass. However, the basic flying principles are still very much the same. While the 767 uses FMS (Flight Management System) for primary navigational purposes (inputting waypoints & making calculations etc) it's still possible to navigate via VOR and also by using visual references. The heading can also be adjusted and maintained in a similar way. These principles are taught at PPL level (Private pilot) and it is relatively basic stuff. The control column and rudder in a 757/767 operate exactly the same as they do in GA aircraft too. Therefore, anyone with sufficient training at PPL level should be able to perform basic navigation techniques and manoeuvres in a Boeing 757/767 without having to spend much time specifically studying the 757/767 controls & systems. It's not as though the Boeing 757/767 uses glass cockpit technology or anything!
The autopilot system is totally different though. However, to make adjustments to the heading, altitude and indicated air speed, you only have to turn three knobs on the MCP. (one for each) How hard can that be? It's pretty intuitive for anyone with a basic understanding. Also, the throttle quadrant becomes pretty much redundant when A/T is engaged, especially during flight, so they didn't need to worry about that. (Though they need to be retarded upon landing) This sort of information is well documented. I've no doubt that these highly intelligent terrorists would be able to perform this after simply studying the aircraft manual & documentation.
A student private pilot who enters the cockpit of an airliner will most likely be overwhelmed by the amount of switches and knobs available to them. In comparison, a Cessna 152/172 doesn't even have a quarter of the number of switches/knobs. However, one can easily familiarise themselves with the 757/767 flight deck by studying the respective flight manual. They will soon discover that most of the switches/knobs are redundant for their needs. Especially when you don't need to make a departure or prepare for an ILS approach or even a landing! (That's the more tricky stuff which the terrorists didn't even need to do!)
There's a number of things the terrorists would have been unable to do. AFAIK, none of them had any IFR training other than simple navigational techniques. If the weather or visibility was poor, they would have been MUCH less likely to pull this off. In addition, without sufficient practice in the respective aircraft, it's also unlikely that they would be able to make a successful landing. Taking off could also be a problem (depending on runway length & aircraft mass) because they would be unlikely to know how to calculate V speeds. The engines are automatically de-rated. Therefore, those calculations and configurations are very important. All of this is what most airline pilots spend most of their training on. The terrorists didn't need to know ANY of that in order to carry out their mission.
I trained for several months in a Cessna 152, 172 and also in a Piper Warrior. When I "flew" in a real life 757 Level D simulator in London, I was amazed by how much I was able to do with the knowledge I attained from my training in a GA aircraft. That said, actually landing a 757 Level D sim was another matter altogether. But in this case, the terrorists didn't need to know how to do that either, did they?
So yeah, I don't think it was impossible for the terrorists to do what they did. That said, it wouldn't have been plain sailing. There's many factors that could have doomed their plan. Though, they did have a lot of time to plan for it!
