How long will it take to complete your first Ironman?

Use our guidelines and a simple formula involving your PBs for Olympic distance and marathon to work out your predicted finishing time

Athletes run on the sand at Ironman Wales

Embarking on an Ironman involves a journey into the unknown, both physically and mentally. Many triathletes relish this challenge; it’s not just the buzz of overcoming adversity, it’s also great for putting the trivial hassles of daily life firmly into perspective!

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But predicting your first Ironman time is an extremely imprecise science. You might think that you can just take your steady-state training pace for each discipline and extrapolate from there, but it’s not as simple as that.

The extraordinary length of an Ironman means that you’ll almost certainly be entering new territory where you’ll encounter new levels of muscular fatigue, severe nutritional challenges, and the possibility of cramps, blisters, dehydration and more.

The art of prediction

The physiological demands of an Ironman have always fascinated sports scientists, explaining why there have been a number of studies into what makes a good Ironman triathlete and what characteristics help predict a fast time.

For example, one study compared the height, weight and body fat percentage of accomplished Ironman triathletes with those of elite athletes from the sports of swimming, cycling and running. It found that the physiques of these triathletes were most likely to be similar to that of elite cyclists.

Of course, while this is all very interesting, these studies don’t answer the question of how a novice triathlete might be able to better predict their race time. However, a 2010 study had a closer look at this question.

In this study, the researchers studied 83 male recreational triathletes participating in Ironman Switzerland in 2009. During the afternoon prior to the race, the researchers took a number of physical measures – height, weight, body mass index and skinfold measurements – to determine body fat percentage.

Nothing unusual there but what differentiated this from previous studies was that the athletes had also been keeping comprehensive training diaries, which recorded data for each training session, including the distance, duration and average speed of sessions. They also gathered personal best times for an Olympic-distance triathlon and a marathon.

At the end of the race, the times recorded for each triathlete were matched to the data gathered for that triathlete. The results were number crunched to see which – if any – variables predicted race times.

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What emerged was that the triathletes’ marathon and Olympic-distance personal best times were related to their Ironman race time. Also, average run speed during training was related, though less strongly so. In fact, statistical analysis indicated that these three variables alone explained 64% of the Ironman race time.

This somewhat surprised the researchers; they had expected that, in line with previous studies, body fat percentage, cycling volume and personal best time in an Olympic-distance triathlon would be most related to the Ironman race times.

OD + marathon = IM

The study above indicates that either marathon or Olympic-distance triathlon PBs could be useful for predicting your Ironman time. But now a new study on 53 recreational female triathletes suggests it might be possible to combine both of these to yield a more accurate predictive formula.

As before, researchers found that the body characteristics and weekly cycling volumes of the female triathletes were poorly predictive of Ironman performance. By far the strongest predictors were previous PBs for the marathon and Olympic-distance triathlon. The data was added to a graph and a line of best fit was calculated. When the data was analysed, the researchers were able to produce a formula to predict long-course race times (all times in mins):

Ironman time prediction = 186.3 + 1.595 × (PB for Olympic-distance triathlon) + 1.318 × (PB for marathon).

For example, if you can run a 3:30hr marathon and have recorded a 3:00hr PB for an Olympic-distance triathlon, your predicted Ironman time would be 12.5hrs. Of course, this is no guarantee of your Ironman time, as statistics can only take you so far. But for novice triathletes who’ve previously completed a marathon and an Olympic-distance triathlon, it’s not a bad place to start.

(Main image: Ross Grieve / Dirty Green Trainers)

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For lots more long-distance advice, head to our Training section