Your goal pace is very important to nail down early on in the program. It’s all about effort and the effort you can sustain for the desired distance. There are a few ways to get a rough idea of what you can do:
Are you an experienced runner?
Experienced runners will typically know what they can realistically aim for.
Predicting Your Time
If you have already competed in some running events then you can use a prediction calculator to measure how fast you can theoretically run other distances. Remember this is just a guideline and is based on your maximum efforts for the distances you enter. http://www.marathonguide.com/FitnessCalcs/predictcalc.cfm
Endurance events are all about the effort you can sustain for various lengths of time. The longer the event, the lower the intensity you can sustain. The shorter the event the higher the intensity you can sustain.
If you are inexperienced or in the category of “want to finish” versus “want to race” then it’s better to be at the lower end of the effort spectrum. Very good runners can stay close to their threshold for 60-120 minutes depending on their level (about 75-85% of maximum effort). Less experienced runners need to be down around 60-65% of maximum effort especially considering the length of time increases for someone who is slower. The longer the event takes you the lower your effort needs to be in order to sustain it.