Greg McMillan is one of the most well known running coaches today. The main reason you should know about his website is for the McMillan Running Calculator. This is one of the best tools a runner can use. Simply plug in your latest time for a given distance and it will predict your time in other distances.
Now there are a few very important points that you must know. These are equivalent times, not necessarily times you will be able to produce. The calculator expects you to be properly trained for the distance. For example, a 20 minute 5k is equivalent to a 3:15 marathon. That does not necessarily mean that if you are training for a 20 minute 5k, you will be able to do a 3:15 marathon off of the same training. It means that with proper training for that distance, you would expect a 3:15 marathon at your current fitness level that produced the 20 minute 5k.
The calculator also provides appropriate training paces. With that 20 minute 5k, it recommends all pace ranges from 400m speed sessions to your recovery run pace.
If you have an iPhone, you can also download the McRun app to have the calculator more easily accessible.