To predict your finish time for the 10K Race using your 5K finish time, you have to multiply your 5K finish time by 2 and add 1 1/2 minutes. Example: If your 5K finish time is 25 minutes, then your predicted finish time for your 10K Race is 51:30 minutes.
To predict your finish time for the Marathon using your Half-Marathon finish time, you have to multiply your Half-Marathon finish time by 2 and add 10 minutes. Example: If your Half-Marathon finish time is 1:45:00 hours, then your predicted finish time for the Marathon is 3:40:00 hours.
To predict your finish time for the Marathon using your 10K finish time, you have to multiply your 10K finish time by 5. Example: If your 10K Race finish time is 50 minutes, then your predicted finish time for the Marathon is 4:10:00 hours
(Source: The Competitive Runner’s Handbook by Bob Glover & Shelly-lynn Florence Glover)
Bald Runner 
I find using the age graded tables give a good indication of predicting times. You can find a calculator at http://www.howardgrubb.co.uk/athletics/wmalookup06.html
If you havn’t used it before, give it a try. Best wishes, Tom
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Thanks Sir Jovie, that’s a useful tool for predicting times, simple but surprisingly accurate !
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this is very good info. and will use it a basis for my next marathon. but i will probably add 20 mins instead of 10 min so the expectations will not be to high
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i guess, the prediction for the finish time is good for flat courses in temperate countries.
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