[Coda]

Pathfinder follows D&D 3.5 mechanics pretty closely. Some details vary, especially when it comes to feats and skills, but in general you can assume that 3.5 and PF are inter-compatible.

Being a GM isn't difficult because of the mechanical work you have to do. Game balance is rarely a concern. If you've played the game enough to be familiar with how it works, you can generally figure out how much of a threat any given mechanical encounter will be, and if it turns out you're wrong you can throw in a couple more monsters to make it more difficult, or intentionally have the enemies not act optimally to make it easier. (Like... maybe have the dragon not use its breath weapon this turn, even though it could if it wanted to.)

No, what makes being a GM hard is that it requires a ton of creativity. It's more about writing stories and building worlds than it is about actually running the game. An experienced set of players will do most of the work of running the game, but that only works if the GM is able to come up with something for them to do.