How To Predict Your Marathon Time

Yasso session? Half marathon time x 2+6%? Or is there a better way?

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how to predict your marathon time

Trying to predict your marathon time is difficult. However, it’s very important to at least try and make a decent go of this – from the relatively minor considerations like what time train to book home and what time to tell your loved ones to be at the finish, to the all-important race day pacing. So how do you do this?

Out With The Old

The original race time predictor was devised by Pete Riegel who used the formula:

T2 = T1 x (D2/D1)1.06 where T1 is the given time, D1 is the given distance, D2 is the distance to predict a time for, and T2 is the calculated time for D2.

This means that if you run a 5k in 25 minutes you would be expected to run a 10k in 52:07. That may sound reasonable if you’re in marathon training as you should be able to complete a 10k very easily. However, the larger the distance the more inaccurate it gets. Say you wanted to run a marathon in under 4 hours and you pushed and pushed through the last few miles, pulling a sprint finish out of nowhere, finishing in 3:59:59 (congratulations, by the way!) would you then be able to run another marathon immediately afterwards in 4:20? I’m thinking ‘no’… For longer distances Riegel’s formula works well for the elites but for mid and rear pack runners, it’s a bit unrealistic. For example, a 1:45 half marathon would equal a 3:38 marathon. That’s doubling your distance and adding only eight more minutes. Unless you’re a seasoned marathon runner, that’s a bit ambitious.

Why Is It Unrealistic?

Marathons take a long time which means there’s a lot of time for things to go wrong – from
weather conditions to your own personal circumstances. You can probably get through an hour’s running without needing a wee. Make that more like four or five hours and your chance of needing a toilet stop, with the possible queuing that goes with it, increases significantly. The longer time you’ll spend running means you’re also more likely to develop stitches, trapped wind, aches and pains and go through bad patches which will require you to slow down. There’s also this thing you may have heard of called ‘the wall’. It exists. At some point from 20 miles onwards your legs will feel like they’re running through sand and the negative voices in your head will be shouting louder than ever. If you lose less than two minutes per mile between this point and the finish, you’ve done well.

The Yasso 800s

Bart Yasso was a successful American distance runner of the 1980s. He devised a training
session which he would do two and a half weeks before raceday. This session was 10x800m with a 400m jog recovery between reps. The time in minutes and seconds he would take, on average, for his 800m reps would be the time in hours and minutes he would take in the marathon, eg an average of 2 minutes 40 seconds for the reps would mean a 2:40 marathon. This is a slightly better way of predicting the marathon time as it’s a pretty tough session which will really test the speed-endurance of your legs. However, it doesn’t work for everyone. I do a lot of track sessions and my powers of recovery are good so I’d be reasonably fresh at the start of each rep. I’d back myself to do this session in about 3:10. Unfortunately I’m definitely not a 3:10 marathoner!

So How Do I Predict My Time?!

There are two studies whose authors have identified the exact same problems I’ve highlighted. The first webpage I’d recommend you head over to is Fetch Everyone. Ian Williams, the creator of the page has spent many hours number crunching half marathon and marathon race times to try and create a link between the two. He’s fed an incredible 30,000 marathon and 57,000 half marathon race times into his database and created a formula which varies appropriately from elites to six hour marathoners. Interestingly he found quite a big discrepancy between men’s and women’s results in that women were more able than men to hang on to their half marathon pace and go on to run quicker marathon times. I’m open to suggestions of why this is but I have two theories; one being that women are conditioned for childbirth and their higher pain threshold makes them more able to handle ‘the wall’, the other being that men naturally have more speed which gives them an advantage over women in the half marathon.

The second webpage to look at is Runner’s World Race Time Predictor The big difference with this one is that it takes training volume into account. If you do a couple of 10-12 mile fartleks every week – and nothing else – you could probably make a decent stab at a half marathon. However, your training would fall well short of what is required for a marathon.

I put my recent half marathon time into both webpages and there was only 11 seconds
difference between the two!

What To Bear In Mind

Both webpages are based on optimum performance. Therefore you may want to adjust your time accordingly. If you’ve done a pan flat half marathon and have entered the Snowdonia Marathon, I don’t hold out much hope of you hitting your prediction. Likewise, if you’ve run a half in the wind and rain and there’s some beautiful cool sunshine on marathon day then you could pleasantly surprise yourself.

If you’re still not sure how fast you can run then err on the side of caution and start slower than your best guess time – particularly if you’ve never run 26.2 miles before. Your 20 or 21 mile training run might have gone brilliantly but nothing can prepare you for those last five or six marathon miles. Remember that in your first marathon any time is a PB and the slower your run the easier it will be to beat next time. People generally don’t care what time you did in your first marathon, they’re just impressed that you’ve done it!

what you need to know
  • Runner’s World Race Time Predictor and Fetch Everyone are the most accurate marathon time prediction pages

  • Training plays a huge part in how you do on the day. The formulae will assume you’ve done at least one or two 20 mile training runs

  • If you‘re reading this within a month of your marathon it’s definitely too late to change your weekly training volume! Be sensible and consider upping your training mileage a bit next time

what you need to do
  • If you have time before your marathon, enter a half marathon on similar terrain to your marathon

  • Put your time on the webpages noted above. Divide your race prediction into miles or kilometres. That’s your race day pace. Maybe you could write your target splits on a piece of paper and put it in a pocket or tape to your wrist on race day

  • If things go badly, e.g. you pick up a niggle and don’t train in the last couple of weeks or if the heavens open as you start, don’t be afraid to increase your target time

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