What do you do with a calling?

Today I have been thinking a lot about callings, mine specifically. I know I am meant for something - many somethings but I don't know what to do with them. I used to think that when you get a calling you have to respond immediately or you miss the boat so to speak. But now I  am wondering about how God prepares you for callings. I have seen people with heavy calls on their heart that they can't answer because the doors never open no matter what they try. I have also seen people who have never known they calling until they accidentally walked through an open door completely unawof what God had been doing all along.
I have a friend who told me today that she knows she is meant to go to cambodia and some other country but she said it could happen when she is 60. She just knows that God will lead her there someday. For now he has other things stirring in her that he has not shown he yet but that he is preparing her for anyway.
I find it interesting that the timing of WHEN God reveals a call is as much a part of the preparation process as every ting else that must happen so you are prepared to want the call, accept it, and to walk through the doors he opens.
I think that I am in some sort of season of preparation for a calling that I don't know about. I think it connects to my "job/career" and allows me to show people Jesus with the things I am talented in. A part of me really wants to just say what the calling is and just work toward it but I am starting to see that it would limit God to do so. Then I could never go farther than I could imagine. I would never go beyond what I could believe is possible on my own strength which would mean I wouldn't depend on God and I would stop preparing.
My prayer for this next school year is twofold. 1) that I will honor God daily by lovig oters with my time, skills and abilities so that I may learn what God has to teach me in this season and 2) that I will continue pursuing God and therefore listening to God about how I should prepare for what is to come. I feel like now is a time to be focused, but God is making it so I have to go to him for my focus and direction. This I know is a good thing to learn but it doesn't make it any easier.