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A very good question.
If you're contemplating running one, even more important. During your training you need to fully be aware of how it feels running that far. You need to test out your legs, heart and lungs.
So plot out your route carefully. Throughout your training, increase the distances you're able to run for and as you get nearer to the event, you need to run the entire distance without rest. Charting your training runs like this will help you get a clearer picture as to how it will feel on race day.
Here's a very useful page which provides you with the answer to how many miles is 10k , but also some invaluable training insights and a great race day strategy.