Tony has been training for the Pittsburgh marathon and someone asked him how he chooses a goal race. He broke it down into five easy steps... 

1. Decide how far you want to race:

You can choose any distance race from a 5K to an ultra-marathon.
5K - 3 miles
10K - 6 miles
Half Marathon - 13 miles
Marathon - 26 miles
Ultra Marathon - 50 to 100 miles
Triathlons - vary between 16 - 140 miles

2.  When do you want to race? AND when do you want to train?
 
When considering what season you want the race to take place (Spring, Summer, fall, Winter) you also need to consider when you want to train. For instance, if you want to do a fall race, your training will take place during the summer. 
 
 How much training time you’ll need will be discussed in an upcoming step. 
 
3. Where do you want to race?
 
Do you want to race locally or are you looking for a destination race? There are a lot of factors that go into making this decision that should be considered (fees, accommodations, terrain, etc.). To locate races you can use these two resources: 
runningintheusa.com 
trifind.com
 
4.  What terrain / conditions do you want the race to be in?
 
This will help determine where you want your race to be held. Think about the terrain that you’re looking for - do you prefer flat runs or do you love running up hills. 
Flat race, hilly race. Also take into consideration the climate / weather of the place you’re considering - is it typically humid around the time of the race? What will the temperature be? 

5.  Assess your current fitness level:

Your current fitness level will not only determine the length and type of race you’re ready for but it will also help you determine how much training you need to prepare for the race. If you’re a beginner you’re going to need an optimal amount of time to train and prepare. Typically you need:  

4-6 months to prepare for a marathon
3-4 months for a Half marathon
8-12 weeks for a 10k
4-8 weeks for a 5