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)