I think there is a "fear" that new GM's go through - the fear of being caught off guard, of not having the answers or response to a player's actions.
You know the truth of it is that it's all a learning process. Overcoming that fear of the GMing chair is learning to overcome that fear of "loss of control" or failure, or whatever, and it's also about cultivating the desire to do a "better" game. And I find learning good basic principles about GMing and making a better game removes that fear, and it also, uncoincidentally, removes "railroading" and "Quantum-Ogres" off the table.
Seriously, making great campaigns requires work. Whether that means work in learning how to GM well - which includes Improv, or doing whatever is "good prep" for you, it takes effort. It *seems* effortless when you have a lot of experience, which a lot of my players assume about me and other GM's of my vintage. But that's because they don't understand the work and effort put in to develop the skills we possess. And it's never perfect. It's NEVER EVER perfect.
Improv is just a paintbrush - how well you use it and to what end is up to you. Lets be real - someone that "railroads" their games is going to do it *regardless* of whether they Improv or not. I actually think learning to Improv, and do it well, may lead to freeing one from the Railroad if they put some of that imagination into it.
Of course I also think nothing gets Rookie GM's to be Journeyman GM's than having Veteran GM's around.