Exact Marathon Prediction Formula

One formula is to take your 10k (road) time and multiple by 4.66. This only works if you have been doing the necessary distance so that you can run a marathon relatively as well as you can a 10K. (a track 10,000 will be a little faster and you might need to tack on a little bit)

An almost identical rule is to add 10% to your pace (1.10 x 42.2 is 46.42, divide by 10k and you have 4.642, marginally faster than the 4.66 rule.

Any rule that adds a fixed number of minutes will have flaws above and below certain pace levels. You might convert that 10 minutes to a percent and have a more reliable rule. This rule, at the listed 76 pace implies a slowing of 6.58%, but a real stud would be slower by over 8%, which is too much of a slow down. A 60:00 guy (especially if not a peak race) could be expected to go as fast as 2:06, which is 10.25% slow down.

Even the 6.58% is conservative because if you use the double-the-distance, increment the time by 6.6% rule you would get a 6.6% going from 10k to 20k (or a HM, similar distance), and then tack on another 6.6%; with compounding, it is 13.6% slower. If you have been doing at least some of your long runs quickly you should be able to do better than this HM rule.