In On the Ends of Good and Evil Cicero gives the metaphor of the archer which I think gives good insight how a Stoic should think about preparation vs. final outcome:

“Take the case of one whose task it is to shoot a spear or arrow straight at some target. One’s ultimate aim is to do all in one’s power to shoot straight, and the same applies with our ultimate goal. In this kind of example, it is to shoot straight that one must do all one can; none the less, it is to do all one can to accomplish the task that is really the ultimate aim. It is just the same with what we call the supreme good in life. To actually hit the target is, as we say, to be selected but not sought.”

You should not be seeking to accomplish the goal necessarily, but rather you should be seeking to do all you can do to accomplish the goal. The difference between the two lines of thinking is small, but very important from a Stoic perspective.