When it comes to define your goals, the SMART-strategy is a useful tool:
Goals have to be precisely formulated. What do you want to reach? What will be the result? Try to be as accurate as possible.
e.g.: I am going to participate September 9, 2015 at the half-marathon in Examplecity.
Goals have to be measurable: In which way, how often and how precisely to you want to measure?
e.g.:I want to run the marathon three minutes under my personal best time.
Goals have to be attractive and motivating.
e.g.: Up to the marathon, I want to lose 2.5 kg of weight through continuous training.
Goals have to be realistic, no matter how motivated you are. Consider your strengths, the tools that are at your disposition and your surroundings.
e.g.: I am going to run 2-3 times a week and switch to a healthier nutrition in preparation for the marathon.
Finally, a defined time frame is essential. Until which day/month/year do you want to reach your goal?
e.g.: I will have finished my preparations until September 8, 2015 in order to participate on September 9, 2015 at the marathon.
Visualize your goals! Listing only the facts and necessities will make it harder to reach your goal.