In case you had not realised, it’s marathon season – and I’m not just talking about the one on the telly – there are at least 30 listed in the Fetch Everyone race finder this month. And, while many runners are happy just to complete this challenging distance, there are plenty who are cramming their heads with calculations, extrapolations and dreams of PB glory.

Unless you are the sort of person who turns up to a marathon in tennis shoes and cut-off jeans, you have probably got a good idea of the sort of pace you are able to maintain over longer distances. But, until you have gone the whole way, it’s hard to know how well you’ll finish. Long runs, fuelling strategies and careful preparation all help, but you’ll find out plenty about yourself in that final six miles.

This article came about because I was always bemused by the predictions that I got from pretty much every online “calculate your marathon time” tool. I ran my best half marathon in 1hr 46m, and most calculators suggest a corresponding marathon time of 3hrs 40m. My best attempt is 4hrs 15m. It was a wonderful experience, but I’ve always wondered where I lost the extra half an hour. I don’t remember stopping for a haircut.

My training could have been a bit better – but it made me wonder whether the race time predictors are giving us a realistic view of what we’ll achieve. How many runners set out to achieve those predictions, and end up having an awful race? Ultimately, predictions help set our expectations and inform our plans – so it makes sense to ensure they are good ones.

The Fetch database holds over 30,000 marathon and 57,000 half marathon performances – enough to expect to find some sort of relationship between them. There were red herrings of course – such as folks who have run a great time at a flat half, then slogged their way around the Beachy Head marathon. But by concentrating on runners who had completed at least five half marathons and five full marathons, I found a core of about 1,000 runners who (you would hope) would have recorded a representative time at both distances. They range in ability from five-minute-milers to 12-minute-milers.

In 1977, Peter Riegel published an article in the US edition of Runner’s World that introduced a formula for estimating race times – and it is this formula that is most commonly used in dozens of race calculators all over the internet. You can feed one race time into it, and get a prediction for how fast you would run at a different distance. It is awesome because it can apply to pretty much any distance between one mile and a marathon – and in most cases, it gives a decent estimate. Here it is:

T2 = T1 x (D2/D1)1.06

D1 = the distance you have already run

T1 = the time it took you

D2 = the distance you are about to run

T2 = the predicted time.

So if you ran a half marathon in 1hr45 (105 minutes), and wanted to predict your marathon time, you’d fill it in like this:

T2 = 105 x (26.2/13.1)1.06 = 219 minutes = 3hrs 39m

And there’s that prediction that left me bemused – a whopping 14% quicker than I managed. But am I alone in that feeling of slight implausibility? Well, no – at least not judging by the data. Allowing a generous 5% margin of error (that’s nearly half a minute a mile for a four-hour marathoner), this formula got it wrong for 65% of the runners in my data set.

Let’s be clear. I’m not saying that there are bad sums at work here. Far from it, I think this is a very adaptable and elegant formula, and I’m sure it was a good fit for the limited data that was available to Riegel in 1977. I’m in a very fortunate position with access to so much data from runners of all standards. Riegel’s formula is also probably a good fit for those of us willing and able to put EVERYTHING into our training. These days we all have busy lives, what with family, work and TV.

What I’m aiming to do is not to invalidate, but to enhance this formula – to make it work for all of us, and to give us something achievable to aim for. Sure, this may even mean that you exceed your targets – but I think that’s better than never reaching them.

Take a look back at the formula. The 1.06 is important – it represents the rate at which we slow down as the races we run get longer. If it was a nice round “1”, it would mean that we could run a marathon at the same pace we would run a single mile. If it was as high as “2”, it would mean that Roger Bannister in his prime would take nearly two days to run a marathon. The higher the number, the more we end up slowing down as distance increases.

Elite runners are capable of less than 1.02 in the transition from half to full. Paula Radcliffe ran her best half marathon at 5:06/mile, and her world record marathon in London 2003 was 5:10/mile.

I fed all of my 1,071 runners through that formula, and found that only 49 of them managed to hold on to the tails of 1.06 – it was far more common to see a score of 1.15. So if we adjust our formula to look like this (for half marathon to marathon only):

T2 = T1 x (D2/D1)1.15

... we see an instant overall improvement. The number of “bad” predictions drops from 65% to just 27%.

But let’s not stop there. We know that faster runners tend to be capable of holding on to their pace for longer. So instead of using a constant 1.15, let’s connect that number to the speed of the runner, using their half marathon time.

I also looked at the differences between male and female performances. We all accept that the fastest men generally beat the fastest women, but you might be interested to know that if a man and woman can both run a 90-minute half, the woman is likely to beat the man by nearly four minutes over marathon distance. That difference gets bigger as pace reduces – women capable of two-hour half-marathons can look forward to beating their male equals by nearly nine minutes in the marathon.

Taking these factors on board, along with plenty of medication, I’ve built a new calculator that will predict your marathon time based on your half marathon. It should predict your time to within 5% in 77 out of 100 cases.

Of course, it’s just a prediction, and within each prediction there’s leeway, and plenty of people who stray from the norm. It’s based on averages, and as a runner you are anything but average. It doesn’t take into account anything that happens on race day, and it doesn’t know anything about how hard or effectively you trained. And therein lies the real secret towards converting a good half marathon time into a great marathon experience.

Try Ian’s calculator at fetcheveryone.com