Always start with WHY... Try to figure out WHY you are taking on whatever challenge you're choosing. Of course, there is usually there is a mix of reasons for doing what we do, but the more we understand our motivations and what we're after, the more honestly and efficiently (and in the end successfully) we can work toward achieving them. This is an especially important question to ask nowadays, in our world of social media and near-constant comparison of ourselves with others.
In our hectic, often overloaded lives we need to be honest about the hurdles in our path to achieving big goals and dreams. Take for example a doctor who is constantly working and on-call and can never get enough sleep. For that person single-minded, high-level training for an arduous endurance event might be nearly impossible at best, and potentially injurious and downright dangerous at worst. That's not to say the doctor should never strive for big athletic endeavors, but it is to say that he or she should be extra thoughtful about how much training they will be able to safely and sanely sustain. Same goes for anyone with demanding jobs: new parents existing on little sleep, folks with crazy travel schedules, and so on.
Appropriate goal-making means putting your intention into both short and long term goals and knowing what is appropriate for each. Short term goals are the stepping stones that lead to the long term ones; they are meant to be difficult enough to attain that they feel like genuine victories that you had to truly work for, but they are not too difficult so as stymie your overall momentum, mentally, physically, or both. Of course, the goals you set should follow a logical progression; by starting small you build up in intensity and duration until you arrive at the big goal, fit, confident, and ready to go. If you try to go too big, too soon in your progression you will only set yourself back and create a trend in the wrong direction.
-When in doubt, stop and check yourself... WHY am I doing this again? Is it healthy? Is it sustainable? What am I sacrificing and what am I gaining?