As a coach this is a question I am often asked. There are many good pace calculators / race predictors out there, including but not limited to Hanson, McMillan or even your Garmin watch.
The table I have pulled together here comes from multiple sources, it assumes perfect conditions, it does not take in account temperature, wind speed, humidity or gradient. It is intended as a guide only. It assumes that you are optimally trained for each distance.
How to read the table look at your 5k, best current time, then read across for an indication of what your 10k, half marathon or marathon time is predicted to be.
So if your best flat 5k time is 28mins and you have a goal to complete a half marathon in sub-2hours you know that you will have to work quite hard to achieve that. Not impossible just hard. If your 5k best time was closer to 26mins, you should be confident that with the right training plan you would be capable of hitting that 2hour half marathon target.
No model is perfect, but they do offer a good rule of thumb for beginners, intermediate and improver (competitive) age grade runners.
The key differentiator here is picking the appropriate training plan to reach your realistic or stretch goal.
If you have any questions at all ask the coach before they tailor your training plan for you!
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