Energy Pacing your Ironman

One of the most difficult aspects, when it comes to pacing a triathlon is the fact that it is a multi-modal activity. It is very easy for an elite 10,000m track runner or 1500m swimmer to ascertain how ‘evenly’ he paced his event. He and his coach simply sit down and look at lap splits.

In the world of triathlon racing, however, it is a little more difficult. What’s the running equivalent of a 200W bike? For the pointy end of the field, is the all-too common 5:30 bike/4hr run an optimal way to race an Ironman? What if I’m a strong biker and a crappy runner? Does that give me carte blanche to take advantage of my ‘strengths’ on the bike, or vice versa for that matter, if I’m a 2:30 marathoner, how much will I slow down for my Ironman run split?

The guy pictured above, James Prescott Joule has some answers.

The first thing to realize is that, by and large, for both bike and run, the body is pulling energy from a single, finite energy pool. A fixed amount of calories or kilojoules, stored as fat, glycogen and protein.

The second thing to realize is that Ironman is an energy limited, not a fitness limited event. In other words, just because you can run a 40 minute 10K or bike 300W for an hour doesn’t mean that you have the ability to fuel this rate (or an arbitrary percentage of this rate) of performance in the context of an Ironman.

No, your best Ironman performance will come from a conscious, even, metering of your energy resources with only slight regard given to your personal strengths and weaknesses.

So, let’s get down to it. What is the run equivalent of a 200W bike split, or the bike equivalent of a 4hr marathon? The numbers may surprise you.

I have prepared a table below, comparing the energy equivalents of a 140-300W bike split for a 60, 70 and 80kg athlete.



The numbers are based on ‘average’ economy numbers of 21% gross economy on the bike and 210 ml/kg/km on the run. In other words, if you have extraordinary run economy due to superior technique &/or muscle composition, your optimal splits may be marginally different, but marginal is the key word.

So, assuming a flat run course, the above represent equal energy splits for bike and run. To put it bluntly, if you are a 75kg athlete, you have no business biking 200W on the bike unless you’ve proven your ability to run <4:00 off the bike. How do you ‘prove’ this? By exceeding these standards in your Ironman.

Truth be told, there is a speed advantage to a slight negative bike:run split (more so for bigger athletes!!) due to the energy on the bike that is ‘wasted’ overcoming aerodynamic drag. Put plainly you get more speed bang for your energy buck on the run where extra energy goes to increasing speed rather than overcoming additional aerodynamic drag. So, the athlete should seek to slightly exceed these run standards.

Looking at the table, it is clear that body type comes into play, with smaller athletes expecting a faster run for a given bike power split. Thus smaller athletes need a better speed reserve/run fitness than larger athletes.

So, what about the ‘strong bikers’ who make the argument, “Well, I’m not a fast runner so I need to make my gains on the bike?” 3 points to these guys:

1. You’re drawing from a common energy pool for both bike and run and if you’re a crappy runner, that is even more reason to leave some ‘gas in the tank’. I’ve seen good runners coast at <8:00/mi on fumes. Sub-par runners don’t have that luxury.

2. Speed benefits decrease as power rises on the bike, due to aerodynamic resistance, while you always get good speed benefit from increasing energy on the run.

3. If you’re a big, strong guy, you don’t have to be a ‘good runner’ in order to pace appropriately. An 80kg guy biking 200W need only pull off a 4:14 marathon (9:40/mi). In other words, enough energy to jog (not walk) the marathon.

And what about the flipside, a very strong runner, say a 65kg, 2:27 marathoner. Let’s name him, Kyle . Obviously, Kyle has the fitness to run a very fast 26mi. But what bike+run distribution will give him his best Ironman time? Or put another way, how close to his open marathon time should Kyle expect to run?

Let’s take a look at some of his test data for some more info we can use to structure some race simulations. ….

As I have previously stated, I generally find that top age-groupers can fuel 11-12kcal of carbohydrate per minute over the course of an Ironman. From Kyle’s previous FUEL test we see that, with his exemplary FUEL profile, this point occurs at 220W(NP) of power on the bike (see below).



Looking at the table, an even distribution of energy from a 220W bike split would result in a marathon time of ~3:10. More than 40 minutes slower than his open marathon time!! And this is a best case scenario assuming optimal fueling on the bike and a marginally better lactate curve than the last time we tested him.

Clearly, there are many athletes from a running background who have more run fitness than they have the energy to use (ditto for the swim, btw). This has big time implications on training – specifically, the amount of training devoted to improving run fitness.

When it comes down to it, if you are looking for your best possible race performance don’t listen to Mr. Hellriegel or Big Jurgen Zack and ‘make zem suffer on ze bike’. Nope, if your overall time matters to you, take the lead from my bearded friend at the head of this article. Use your energy wisely.

Race smart,

AC