Sometimes You Simply Need to Run

Here’s an interesting case study: one of my athletes, racing at Ironman UK, all going well, until just over halfway through the marathon he starts to slow. Must have biked too hard, right? That’s the obvious conclusion. More often than not, when I review data from an Ironman in which an athlete suffered in the run, I find pacing errors on the bike and when I don’t the next logical conclusion is a nutritional mistake. I, and many other websites, frequently espouse the view that the bike is critical to successful Ironman running and I support this in my training programs with the emphasis tending to be on bike fitness. But my athlete didn’t bike too hard, and thanks to his near compulsive levels of data recording I can be confident he ate more than enough. What happened?

We can start with the swim as, unusually, I have data. If there is one benefit to the Polar system it can record heart rate during the swim; I may not get unreliable GPS plots or stroke counts, but I have some measure of effort. The athlete in question has a strong swim background, to the point that when we first worked together it was a neglected area of training, that changed this year as we concluded that while he could swim up front without much training, it came at a high cost early in a race. Since then he’s increased his swim volume and also made greater efforts to hold back during the swim portion of the race. As the graph below shows, he did go out a touch hard, but had the presence of mind to bring this under control and have what he described as his easiest Ironman swim ever. Perhaps there was room to ease back further for a lower HR, but it’s hard to tell the impact this would have had on his race, recent analysis of Roth results showed how relatively weak the correlation between swim and run performance was.

Heart Rate During the Swim at Ironman UK 2012

We’d developed a clear power-based plan for the bike, using two caps thoroughly practiced and determined in training. Power has been a revelation for this athlete, since investing he has completely eliminated any tendency to surge and learned to control his racing in a narrow band of effort. As the graph below shows he did a superb job of pacing, if anything given his FTP he could have gone harder. The bike was described as comfortable, and with an intensity factor (IF) around 0.69 it should have been.

Bike Power Output Compared to Pacing Strategy at Ironman UK 2012

At this point we should consider nutrition. In planning this race I had questioned the athlete’s strategy – he was proposing an unusually high intake of food, easily at or above the 90g/hour peak absorption level I’ve previously referred to. Again this had been tested in racing and training, his nutrition at Wimbleball has been at a level above what I would use in an Ironman and caused no issues at the higher intensity of work. At Ironman UK he consumed a combination of gels and bars that should deliver between 400 and 500 calories per hour, and no, he didn’t experience GI distress at any point in the day. Fluid and nutritional intake doesn’t seem to have been the source of any problems, and yet two hours into the run things went wrong.

Heart Rate and Speed During the Run at Ironman UK 2012

If there’s a downside to the Polar system it’s a limited amount of memory, meaning thirty kilometres through the marathon the watch stopped recording. In this case that’s far enough in to see where things fall apart. We actually see a slight slowing earlier around ninety minutes in, at that point he still felt good, but early signs of problems are there. The two hour mark sees a shift, he starts taking walk breaks and the pace slows, then thirty minutes later the pace completely drops off. At that point he felt like his legs had nothing to give – empty. I suspect there is refinement to be made here – initial run pace could be lowered, although it felt easy and comparable to training runs, the heart rate might be higher than could be sustained. There’s nothing glaringly wrong throughout the day, no obvious point where he did the damage, perhaps it was an accumulation of small mistake, but nothing stands out from numerous successful races I’ve seen.

As we discussed the issue and looked back over his training logs we started to wonder if there was another, simpler conclusion: perhaps he simply didn’t have the training to sustain that effort for an entire race. Weekly volume is typically around ten hours, something I’ve referred to as minimal Ironman training, and more notable is the dominance of cycling in the week. Partly my emphasis, partly the quirks of athletes that often favour one sport over another. Over time a deficit can accumulate in particular areas. He’s swim fit, he’s bike fit, but was he run fit to sustain a full Ironman marathon at that pace? Possibly not. On average he would run for around three hours per week, not enough – I suspect – to perform at his best after eight hours of racing. Theoretically the race was appropriately paced based on thresholds, what may have been missing was factoring for his volume of training; run targets should have been reduced due to lower run fitness.

He should simply run more. Except, of course, there’s a reason he trains around ten hours per week – life. He can consistently manage ten hours per week, but there isn’t room to regularly do more. We started to discuss other options, perhaps specific run weeks within the schedule or my preference at this point in the year – with only Ironman Wales to go – is to do some specific run blocks over winter to help him develop run fitness. Biking has come on strongly, swimming has never been an issue, making running the focus for a few months could pay back strongly next season and not require him to step beyond those ten hours per week. Sometimes you simply need to run.

Incidentally, I should add that despite the critique this was a personal best for the athlete in question; we just wanted more and it’s important to look at how we might improve on this result.

Ironman Training Library

From nutrition to pacing - a collection of CoachCox blog posts focused specifically on Ironman training and racing.


  • do you think the heat played a part in your guy slowing down , a few of my mates who did it complained about the temperature . How much warm weather training can we get in the UK !

  • I guess the good news is that its much easier to fit in extra run volume than anything else – I find that frequency of running rather than volume helps. Getting out for 20-30 minutes a couple of times a week is much easier to schedule than extra trips to the pool, etc.

  • Paul,

    Hard to say (he might answer it himself here), there were no specific concerns about heat, though it was noted later in the run that he developed a dry mouth and found himself having to drink more. Prior to that he was well hydrated through the bike and into the early part of the run. Possibly this plays a role. I think dehydration is often overstated as a factor, but may have an impact.

    Overall my feeling is there are tweaks – small changes throughout the race that might – I’d emphasise might – make a difference in the outcome, but the single factor that I think limits this athlete is time to train in order to sustain the target pace we had set. Had he slowed the run further from the start he may not have experienced the race falling apart as it did, but I suspect the pace would have placed him only marginally faster. As an athlete I tend to view him as quite anaerobic, stronger at short/hard work with a relatively steep fatigue profile that we’ve worked to overcome, just harder to do for the run when it’s a relatively minor part of training.


    Yes, it should be. Though how much more this athlete could consistently squeeze in is the issue. Definitely needs a period of simply running more and developing fitness there over swim and bike which are ahead of the run.


  • Nick B

    Happy to shed some more light on the heat/ hydration issue as its an excellent point. Unlike some, I started the run in good shape hydration wise. I needed a pee twice on the bike and again 45 mins into the run – happy with that. But as you said, the temperature on the run really ramped up and I started to get really thirsty. The issue was the aid stations were as far apart as they could be at opposite ends of the looped sections. I’d walk through and down 4 full cups but still end up with a horrible dry mouth 5 mins later. It certainly made the second half more unpleasant and I was gagging for a drink by the time I arrived at the next aid station. That was my only symptom of the heat I remember. I think heat/ dehydration may be a contributory but certainly not a causative factor. The wheels seemed to come off far too quickly for that. The HR graphs point to a more gradual decline in performance on the run, but my perception was one of a sudden implosion in speed/effort! Try as I might, the HR never got above 128 after that.

  • RobQuantrell

    Great post Russ.

    Gordo has written about this:

    “if aerobic threshold endurance is equal to or less than anticipated race duration then spending any material time above aerobic threshold in a race will likely prove costly. This seems to be an explanation for why many well-nourished and well-hydrated athletes simply run out of gas. I think this is the most common form of “bonking” and it has nothing to do with race nutrition or hydration. The athletes are just tired.”


    Seems like your intial analysis that Nick wasn’t “run fit” enough to cope with an IM marathon seems spot on. 3hrs a week isn’t even covering the race time. Life and an ideal IM performance are sometimes incompatible!

  • Nick,

    Thanks for clearing that up, guess I don’t need to maintain your anonymity now (though I’m not editing the next post I’m about to publish, too late now, you’ll remain ‘my athlete’ in the text). I agree with what you say about dehydration being contributory rather than causative; there are plenty of little details that perhaps, if performed differently, might have led to small improvements in your result, every race has them. In Bolton, you basically hit the limit of your training, you raced well until the point where as Rob (and you) have referenced you exceeded what Gordo calls your Aerobic Threshold endurance. You might like the post I’m going to make shortly, another set of IMUK data to consider.


  • Rob,

    Funnily enough Nick referenced that very idea from another of Gordo’s articles ( in an email to me last week. It basically applies – we reached a point where the quality of training was good, fitness was good, but the endurance wasn’t quite developed to the point he could carry himself at the appropriate pace through to the end of an Ironman. The limiter there is training time, it’s one Nick and most age groupers have and it probably limits their rate of improvement at Ironman more than anything. I’ve no doubt Nick can improve, but with limited training time he may not do so at his fastest potential rate.

    For working age groupers developing as an Ironman truly is about the marginal gains that come from consistent, focussed training.


  • Dan

    Would be interesting to see the difference in run volume between the athletes in question.

  • Dan,

    I can get fairly precise figures on my own athlete, but if you’ve read the blog on Roger’s performance at IMUK that followed this, I only have his rough estimate of building to 5-6 hours per week of running and around 80km. Figures like those suggest Roger is running at least twice the volume of my athlete here. If you consider Nick’s training volume in light of roughly a 10 hour per week limit then a division in the order of 1.5 hours swimming, 5.5 hours biking and 3 hours running isn’t unreasonable. As I say, it’s quite minimal in many respects, but probably covers the essentials to get round and if sufficient fitness and experience is present to perform reasonably well.

    I see this as a genuine difficulty for ambitious age groupers – time limits may prevent them achieving their goals, or at least reaching them as quickly as they might hope. Nick supported his consistent weeks with a training trip and occasional bigger weekend which will also have helped him perform on race day. Focusing on running for a while during the off season is another trick we can use to address the limitations in training time and run volume.