The best place to start is with a detailed self-assessment. This includes not only identifying your physical limiters, but also mental skills, race execution, planning, and pacing. This self-assessment should also include strengths to exploit and areas that offer the most opportunity for development. Remember that a great many things must converge to get the best race out of you. Don't look at yourself one dimensionally; training is just one facet of achievement, often with too much emphasis placed upon it.
Genetic propensity is something athletes often overlook or choose to overlook. For instance, a very large male athlete of 190 pounds will have a handicap in thermoregulation and climbing ability from the start line. Yet these athletes often choose hot/hilly races that do not suit their physical makeup. Your peak races should accentuate your strengths, not exploit your weaknesses. Figure out not only what sport or distance you are best at, but also what courses will be most favorable.
Tactically planning out what you want to achieve in each season is a great place to start. Carefully planning out both peak and peripheral events is perhaps the most important element of a successful season. Each race choice has implication upon the next race, and the season.
There should be a primary objective for each season. Beyond that, a three- to five-year plan is essential for building an athletic career. Athletes often train hard from season to season with no real or concrete goal on the horizon.
Recognizing the Window
In my opinion, once an athlete enters the realm of being highly competitive, they have about a maximum shelf life of about five years. There are, of course numerous exceptions, but the physical and mental toll of highly structured training and racing cannot be sustained indefinitely. Athletes often need a hiatus and may return to the sport realizing that their best race is now behind them.
Understanding that the window of achievement is a short one is important, but conversely coming out of the gate too hard and too early may prematurely end a promising career. I believe this is especially the case with ultra-endurance athletes competing under the age of 25. These athletes should be primarily focusing on building speed and skill, yet they compete in events that have high risk of injury, high risk of burn out, and are often not sustainable in a typical lifestyle.
A fast athlete can always be trained to go long, but building speed has a much, much smaller window within which the athlete has the capacity to not only withstand the intensity, but recover quickly. Young athletes should spend years developing fundamentals, a strong biomechanical foundation, and an appropriate amount of training volume.