It is not my place to tell anyone what to do or how to behave. All I can do is help you understand yourself better and remove the blocks that stand in the way of you getting what you want for yourself.
When you ask me what would be the best thing to do, it sounds like you are asking me to give you permission to misbehave. You know what the proper thing to do here is--nothing. And you don't need me to tell you this. You can feel anything you want, you can dream and fantasize, but you know that this guy is 'hands off.' He's married and belongs to someone else.
It is normal for young women to develop crushes on older men. And, you need not be guilt-ridden over your feelings and desires. So long as you are clear that you can feel a desire without acting upon it. You could also transform the feelings you have into a learning tool: detail what you like about him, so that you can make sure to find someone similar to him who is available.
Have a happy life!