Joss Whedon, a writer for who I have the highest respect, once stated that in a way having a show that could be canceled at any time actually helps a writer. "It doesn't help your marriage. It doesn't help your digestion, but one thing it does help is your story telling." The reason being that it forces you to consider the most important story you have to tell and focus on that because there may not be a tomorrow. That was the case for Whedon's short lived SciFi series Firefly, one of the best written shows I have ever seen.
I bring this up because games can be a similar writing environment. People move, players lose interest, and gamers want to try new games. All of these things can cut your campaign short. So if you have a great idea one that you are sure your players will absolutely love then don't wait. Think of your game as a show that could be canceled at any time, and act accordingly. What is the most important story aspect of your game? Focus on that because you never know if you'll have time to do so later.
Ironically it was a lack of following this advice that doomed Whedon's current series, Dollhouse. A very cool concept that got bogged down with trifles and filler episodes in the first season. The second season on the other hand (and really this starts with the 13 unaired episode which featured Felicia Day from the Guild) has no real throw away episodes but instead focuses hard on the most important story. I am convinced that had Dollhouse started off as strong as it is now it never would have been canceled, but hey it's FOX so it probably would have gotten the boot anyway.
Dollhouse had a long set up, and I'm not saying that a steady build up can't be a good thing. But getting off topic with a series of unrelated, and unimportant, meaningless tasks and distractions doesn't add to the overall story of the game. When designing an adventure think about how you can tie it in with the overall story arc. Stay focused and I promise you your players will thanks you.