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How To Pace an IRONMAN Run

Essential advice about the optimal running pace, heart rate and perceived exertion to run a fast IRONMAN marathon.

Did you realise that by the time you start the marathon run in an IRONMAN triathlon, you’ll have already been racing from anywhere between 5 hours 30 and 12-hours?

You’ll be tired and low on energy by then, so your pacing strategy needs to reflect that. The best outcome you can realistically expect in an Ironman marathon is to maintain an easy or steady pace throughout, without having to stop or continually walk. That is the secret to achieving an excellent IRONMAN result and in this blog, we’ll show you how…

What is The Perfect IRONMAN Run Race Pace?

There is precious little research on the ideal pace for an IRONMAN but there is one particular study that sets the tone of this blog. It appeared in the journal PLoS One and was based on athletes who competed in one of the world’s most challenging ultra-marathons, the Tor des Geants in Italy. This gruelling event involves running 200 miles over mountains, with 24,000 metres of elevation change. The researchers compared these hardcore athletes with another group who’d taken part in a shorter Alpine ultra-marathon – a mere 103 miles in length.

The findings showed that runners in the longer race had lower levels of muscle damage and inflammation, despite the fact that they ran almost twice the distance as those in the shorter event. The researchers were led to conclude this: “Such extreme exercise seems to induce a relative muscle preservation process, due likely to a protective anticipatory pacing strategy during the first half.”

How does this help you?

Well, it implies that the athletes in the longer event paced it more carefully from the start, compared to those doing the shorter event. And that even with distances of 100 miles and 200 miles, small differences in exercise intensity can make a big difference to your muscle fatigue levels.So bearing that in mind, here’s how to set yourself a realistic pacing plan, using three different measures…

1. IRONMAN Run Heart Rate

You should start your IRONMAN run at 75 to 80% of your maximal heart rate. For example, if your max heart rate is 160 beats per minute, start running at 120-128 beats. Sounds low right? Keep this up until halfway and then you can increase your pace slightly if you feel fresh enough (but the chances are you won’t).

2. IRONMAN Run Perceived Exertion

Based on your perceived exertion your IRONMAN race pace should feel like a 2 or 2.5 out of 10 on the intensity scale, with 10 being your all-out maximal effort. On a good day, this might be described as being an “Easy” pace.

3. IRONMAN Run Pace

If you know your threshold running pace (your current 1-hour race pace) you can use our online training zone calculator to estimate your IRONMAN run pace. Your IRONMAN run pace will be at the lower end of your Zone 2 or even into your Zone 1 pace. You can test your threshold running pace like this.


Just make sure you base your IRONMAN pace on real results done in the last 8 weeks. DO NOT GUESS!!!

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Final IRONMAN Run Tips

Use all three of the methods above as none of them is completely reliable in isolation. There are many variables that can affect them, such as wind direction, gradient and cardiac drift (an increase in heart rate due to heat stress).

The risk of starting too fast is far greater than the risk of starting too slow. You can always increase your intensity at the halfway point if you feel great. Whereas if you go too hard from the start and blow up, you’ll never recover the lost time.

Your IRONMAN run pace will be determined by how hard you went in the bike and swim sections – and your nutritional strategy. If you cycle too fast you will struggle to run the entire marathon run. If you pace the bike section intelligently and get your nutrition right, the chances are you’ll run well. Check out my blog on IRONMAN bike pacing. Or my blog about IRONMAN nutrition.

The longer you anticipate you’ll take to complete an IRONMAN, the lower your intensity should be. For example, if you think you’ll take 15 or 16 hours in total, you should aim for the lowest possible end of the pace, heart rate and exertion suggestions above.​

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Phil Mosley (Coach & Founder)
Phil Mosley (Coach & Founder)

Phil is a recognised endurance expert who founded MyProCoach in 2010 to offer smart training plans that still leave quality time for your family, friends & career - complete with highly-rated coach support (he has sold over 40,000 so far). Learn more here!

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The feedback has been outstanding (rated 5.0 from 100’s of testimonials), and recently we’ve also added further personal coach time… For example, our coaches now check your personalised training zones are set up correctly, and also keep you accountable by getting in touch if you haven’t logged any training for 2 weeks.

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