## How To Predict Your Marathon Time

Can your recent half marathon predict your marathon finishing time?

I’m sure you’ve all seen these prediction calculators online – typically, you’ll enter your times over distances X and Y, and then a calculator will pop out a prediction for distance Z. For instance, one of these (called the Riegel formula) calculates your 10km time by taking your 5km time and multiplying it by 2.085. A 25-minute 5km would predict a time of 52.07 for 10km. In my experience, that’s a pretty good prediction for most people.

Another simple equation multiplies your half-marathon time by the same factor – 2.085 – to predict your marathon time. But this is where the model starts to falter: a two-hour half-marathon runner, for instance, would have a predicted marathon time of 4:10, which I suspect is unlikely for the vast majority of two-hour half-marathoners. My gut tells me 4:30 would be a more likely target.

In this case, using this type of calculator would be a disaster, because you’d start off at an unsustainable pace, and then probably blow up in the last 10km.

**IMPROVING ACCURACY**

A group of statisticians have attempted to improve the accuracy of calculated predictions. To do this, they surveyed thousands of runners, recorded their best recent times over a range of distances, and then ran some statistical models to develop a more stringent prediction.

They found that the 2.085 factor works when going from 5km to 10km – so you can keep using that one. But for a marathon, you should multiply your half-marathon time by 2.19, not 2.085. So our two-hour runner would now be aiming for 4:23. I think that’s still slightly fast, but at least it’s closer.

Still, their predictions won’t be perfect, because they didn’t take into account gender, age, and both long-term and short-term training history. But even if they had, accurate assessment of these factors can be difficult.

**THE REAL VALUE**

In my opinion, these prediction tools can be used ‘diagnostically’ to help you identify your strengths and weaknesses, because you can compare your progression of times across a range of distances to that of the ‘typical’ runner.

For example, let’s say you’re a 25-minute 5km runner, which means your predicted finishing time for 10km is 52:07. But when you attempt to run this time, you find the pace is too demanding. At 7km, you falter, and end up running a time of 53:45 instead.

What lesson does that teach you? I’d say you’ve discovered that your 5km is (relatively) stronger than your 10km, because you haven’t been able to follow the typical progression of time as you’ve added distance. Therefore, your endurance is weak. If you want to address that, you need to focus on longer runs, strength training, longer intervals, and aligning your 10km ability with that of your 5km.

Similarly, if you go out and run a 51:20 – much faster than predicted – then you’ve diagnosed a weakness in your speed over shorter distances, in which case it’s time to tackle some training that will improve your speed.

For the marathon, the implications of not meeting the prediction are even more stark. Which explains why many runners with decent 10km and 21km times fall horribly at the marathon, finishing in a much slower time than predicted.

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