Perhaps the difference from just "being in the world" and missional living is intentionality. I think many times we practice the being in the world part but for get the clarifier "but not of the world". "World" of course stands for something a little deeper than the big blue planet we live on. It is mindset, a way of living, a philosophy of life and actions that bear out that philosophy.
I am thinking that true missional living is best defined by Jesus' command for us to "let your light so shine before men that they may see your good works and glorify your Father in heaven" (Matt. 5:16).
How do we let our light shine? May I suggest it is as simple as imitating Jesus? I know we are supposed to live like this always but the truth is we don't. How about a three day challenge? Would you be willing to yield to His Spirit and make the attempt to live like Jesus, let you light shine in the world while not being of the world for three straight days? Would you intentionally live like Jesus for the next three days? I suggest we try an hourly "check-in" with the Father to see how we are doing.
You let me know how it goes with you. I'll let you know how it goes with me.