You can advance the story right to the very end, then walk away and go back to exploring the wasteland. The power to finish everything is in your hands, however awkward it is that everyone sits there and waits patiently for you to return some days or weeks or months later.
But look, enough about the end of the game, you'll get there when you get there. And you WILL get there, because once you start playing Fallout 3 you won't be able to put it down until you play it all the way through. So go check it out for yourself, it's this season's must-play game.