How to Use Run Power Data to Pace Ironman Triathlon Marathons

Run with Power by Jim Vance

by Jim Vance

Republished with permission from the article “Perfecting Training for the Ironman Run with Run Power Data” with permission of Jim Vance.

One of the most popular questions among Ironman® triathletes, (and generally the hardest to answer), is “How fast can I run off the bike for the marathon?”

If you’ve ever done an Ironman, you know the marathon pace is much slower than you could run if you were fresh, running an open marathon. But what is that race pace for you off the bike? How can you determine it?

The truth is, you can’t really determine your Ironman marathon race race pace very effectively, because the pace you run off the bike will be greatly determined by the bike intensity and duration, your nutrition and pacing, as well as the course and conditions. All we’ve been able to do as coaches and athletes is train some type of aerobic intensity, and hope that it translates on race day. The problem has been we’ve had to use pace to try and identify intensity, and those don’t match since the pace is slower in the Ironman run but still a hard effort. But by using running power, now you can specifically train for the intensity of the Ironman run, no matter the pace.

Two professional triathletes at Kona, Matt Russell, (12th), and Lionel Sanders, (29th), wore the new Stryd power meter device, a footpod which measures the work rate, in watts, of each athlete while running. (If you’re in need of a refresher on how run power meters work, you can read here.) There are some things which can be learned from these athletes and their power files from the Ironman run.

Matt Russell 2016 Ironman World Championships Marathon Running Power Data

Matt Russell Ironman Kona 2016 Marathon Running Power File

Matt Russell paced himself quite well, in route to a 2:54 marathon. His first half of the race was at an average power of 321 watts, with the second half at 300 watts, The Variability Index of his race was only 1.01, so he ran a well-paced marathon overall.

(Note: since we don’t know Matt’s or Lionel’s functional threshold power for running, the rFTPw, metrics like TSS and IF are not accurate in these files.)

Matt Russell Ironman Kona 2016 Marathon Running Power File

In fact, Russell’s highest 1-hour power doesn’t even begin until about mile 2 of the run, through the top of Palani. That’s good pacing early on. You can see on the downhills, his power drops dramatically, taking advantage of gravity. His uphill efforts are not major surges in wattage, meaning he has run well to get up the hills with minimal energy use.

Lionel Sanders Ironman Kona 2016 Marathon Running Power File

Lionel Sanders 2016 Ironman World Championships Marathon Running Power Data

Lionel Sanders didn’t have the run he is capable of. His first hour was his highest power output. You can see that after the climb up Palani, he really began to struggle. The first hour was 319 watts, but the overall average for the whole run was 270 watts. That’s a 49 watt drop-off, or about 15%. For comparison, Matt’s drop off was less than 4%. The Variability Index of Lionel’s run overall was 1.04, compared to Matt’s 1.01.

Lionel Sanders Ironman Kona 2016 Marathon Running Power File

How to Use Running Power Data to Improve Your Training

So how can athletes use this run power data? Great question! One immediately clear way for athletes to use this is to train more specifically for the Ironman run, and more effectively. Let me explain further.

Much like an athlete can train on a bike with power–using Zone 2 power workouts to build Ironman specific bike fitness–now an athlete can train specifically using run power to build Ironman run fitness, instead of trying to guess at their race intensity using pace or heart rate.

In my new book, Run with Power, I present the key concept of power for running, the speed per watt concept, which I call Efficiency Index, or EI. EI represents how fast you can run for the watts you produce. Much like cycling, we have power zones associated with different intensities and energy systems. The Ironman run is anywhere from Zone 1 to Zone 3 intensity, depending on the speed of the athlete.

For the Ironman run, most athletes would be well served training a lot at their Zone 2 power for running, such as in their long runs. With this type of training, seeing improvements in EI show the athlete that their race specific run fitness is improving. Add heartrate data to this, and athletes can get a very good measurement of their aerobic fitness, based upon the speed per watt at certain heartrates.

For Matt Russell, he ranged mostly between 300 and 320 watts through the race. Long runs at that wattage range, and how well he can maximize the pace of his runs at those watts, can be a huge tool for him to build specific fitness to run well off the bike.

For Lionel, he can probably use this data to determine if the early watts were too strong, comparing the zone he was running in for those early watts, and he could use the power meter to better pace himself in the future, finding the right watts to run at.

Power has the potential to revolutionize run training and performance, much like it has done for cycling, and helping define the specific intensity for the Ironman run, is just one example. If you’re using power for cycling, it’s time to take the next step and run with power.

Jim Vance is the author of Run With Power and Triathlon 2.0, and is an Elite Coach for TrainingBible and SuperFly Coaching. You can learn more from Jim at

Run with Power by Jim Vance

RUN WITH POWER is the groundbreaking guide you need to tap the true potential of your running power meter like SHFTStryd, or RPM2. From 5K to ultramarathon, a power meter can make you faster—but only if you know how to use it. Just viewing your numbers is not enough; you can only become a faster, stronger, more efficient runner when you know what your key numbers mean for your workouts, races, and your season-long training. In Run with Power, TrainingBible coach Jim Vance offers the comprehensive guide you need to find the speed you want.