Why Being Fit Enough to Run the Pace Isn’t Enough

You can run the pace.

You’ve done it in training.

Maybe you can comfortably run a few miles at your target half-marathon pace. Perhaps you can hit marathon pace during a long run. You might even be able to run faster than race pace during intervals.

Then race day arrives.

The first few miles feel controlled. The pace is exactly where it should be.

Later, the same pace starts to feel completely different.

Your breathing gets harder. Your legs feel heavier. Your effort climbs. Eventually the pace begins to slip.

So what happened?

It is tempting to assume you simply weren’t fit enough.

But there is another possibility.

The fitness you can demonstrate when relatively fresh is not necessarily the performance capacity you still have available after prolonged running.

That is where the idea of durability becomes useful.

Being able to run a pace is only the first test

Most traditional measures of running fitness tell us something about what you can do when you are reasonably fresh.

Your VO₂ max.

Your lactate threshold.

Your running economy.

Your ability to complete intervals at a particular pace.

These things matter.

But races are not normally completed while fresh.

As running continues, some of the qualities helping you produce that pace can begin to deteriorate.

One runner might become less economical.

Another might struggle to maintain force production.

Someone else may experience a substantial rise in cardiovascular strain.

Another runner might simply have started at a pace that was never realistically sustainable.

That is why durability is not best thought of as another single fitness quality alongside VO₂ max, threshold or running economy.

It is more useful to ask:

How well can you preserve the qualities that matter for your performance as fatigue accumulates?

Recent research has started to examine exactly this.

In one study of marathon runners, several traditional performance markers were measured while the runners were fresh and again after 90 minutes of running.

VO₂ peak and the speed associated with lactate threshold deteriorated after the run.

Running economy, however, did not significantly worsen across the group.

Different qualities can therefore deteriorate by different amounts, and two runners with similar fresh fitness can still respond very differently to prolonged running.

Two runners can have similar fitness but very different races

Imagine two runners preparing for the same half marathon.

Both can run 10 km in about the same time.

Both can complete several miles at their intended half-marathon pace.

Their fresh fitness looks similar.

But their long-run preparation has been very different.

Runner A has regularly built towards longer sustained runs.

Runner B has done plenty of shorter running and quality interval work but very little prolonged running.

For the first hour of the race, there may be little between them.

Later, Runner B begins needing progressively more effort to maintain the same speed.

There is some interesting evidence here.

A 2026 study compared well-trained male runners with similar 10 km performances but different training backgrounds.

Those regularly completing runs of at least 90 minutes showed a smaller deterioration in running economy during a 90-minute run than runners whose training runs were all under 70 minutes.

Their longest weekly run and overall training volume were also associated with better running-economy durability.

That does not prove that everyone simply needs longer runs or more mileage. It was a cross-sectional study.

But it supports a sensible coaching idea:

Preparing to perform well after prolonged running probably requires some exposure to prolonged running.

That does not mean turning every weekend into an endurance test.

It means preparing specifically for what your event requires.

Running economy can change when you get tired

Running economy is essentially the energy cost of running at a particular speed.

We normally measure it while someone is relatively fresh.

But for some runners, that cost increases after prolonged running.

The pace may be the same.

Producing it has become more expensive.

That can contribute to late-race deterioration.

But it does not happen in every runner or every study, which is why fresh-state running economy only tells us part of the story.

A better question is:

How well does your economy hold up during the type of fatigue that matters for your event?

Strength may matter too

There is also emerging evidence that strength training may help preserve performance after prolonged running.

A 2025 randomised trial took 28 well-trained male runners and added two strength sessions per week for 10 weeks to one group’s normal running.

The strength work included maximal strength and plyometric training.

After the intervention, the strength group showed better running-economy durability at the end of a 90-minute run than the runners who continued with running alone.

Their high-intensity running performance immediately afterwards also improved.

That does not mean:

“Do strength training and you will stop fading in races.”

It was a relatively small study involving well-trained men and a specific strength intervention.

But it does add another reason why strength work should not be viewed purely as injury prevention.

For some runners, improving the ability to produce and preserve force may form part of preparing them to perform while fatigued.

Internal link: Strength Training for Runners: What the Research Actually Says

What about your running form falling apart?

This is another common explanation.

Someone slows late in a race.

Their stride becomes shorter.

Ground contact gets longer.

Cadence changes.

They look less springy.

It is easy to conclude:

Their running form broke down, which caused them to slow.

Sometimes mechanical changes may genuinely contribute.

But the relationship is not that simple.

A recent wearable-sensor study examined 297 marathon performances and found that runners who slowed showed changes including shorter steps, lower step frequency, increased contact time and reduced vertical stiffness.

That sounds like classic “form breakdown”.

But another 2026 marathon study found that much of the apparent biomechanical deterioration disappeared once researchers adjusted for the fact that the runners were moving more slowly.

So at least some of those mechanical changes may be a consequence of slowing, rather than necessarily the original cause.

If someone’s stride looks different at mile 23, I would not automatically prescribe technique drills.

First I want to know why the runner slowed.

The marathon wall is a durability problem — but not just a carbohydrate problem

The most obvious example of this whole discussion is the marathon “wall”.

Someone can feel relatively controlled for 18 or 20 miles and then experience a dramatic deterioration.

Pace falls.

Effort rises sharply.

The legs can feel completely different.

Carbohydrate availability matters here.

Running a marathon places substantial demands on glycogen stores, and inadequate carbohydrate before or during the race can contribute to a major late-race slowdown.

That is why planned marathon fuelling matters.

Internal link: How Much Carbohydrate Do You Actually Need During a Run?

But saying someone hit the wall simply because they “ran out of glycogen” is probably too neat.

Recent reviews increasingly describe the wall as multifactorial.

Carbohydrate availability interacts with pacing, muscle fatigue, cardiovascular strain, heat, hydration, gastrointestinal problems and perceived effort.

Run the first half too aggressively and carbohydrate use rises.

Muscular fatigue may accumulate faster.

Cardiovascular strain can increase.

The pace that initially felt comfortable becomes progressively more expensive.

Eventually maintaining it becomes impossible.

So the useful question is not always:

“Was it pacing, fuelling or muscular fatigue?”

Quite often, several factors are interacting.

But not every pace failure is a durability problem

There is also a danger of taking this idea too far.

Imagine somebody training for a 10 km.

They can run 5 × 1 km at 5:00/km with recovery between each rep.

They decide their target 10 km pace should therefore be 5:00/km.

On race day they reach 6 km and can no longer maintain it.

Was that poor durability?

Possibly.

But there is a more basic question to answer first:

Was 5:00/km ever a realistic continuous 10 km pace?

Being able to repeat a pace with recovery breaks is not the same as being able to sustain it continuously.

Before trying to improve fatigue resistance, we need to establish whether the target itself is realistic.

That becomes particularly important over shorter races such as 5 km and 10 km.

Durability is generally most useful when thinking about prolonged endurance exercise. It should not become a complicated explanation for every situation where somebody simply starts too fast or chooses an unrealistic target.

So how do you actually train durability?

I would not start by trying to “train durability”.

That is too vague.

Instead I would ask:

What tends to deteriorate for this runner, in this event?

Then work backwards.

1. Identify the failure signature

What actually happens when performance drops?

Does pace suddenly collapse?

Does it fade gradually?

Do the legs become the obvious limiter?

Does effort rise dramatically while pace barely changes?

Does the problem happen mainly on hills, in heat, or only beyond a particular duration?

We are looking for more than:

“I got tired.”

Everyone gets tired.

We want to know how that fatigue shows itself.

2. Check whether the pace was realistic

Before blaming durability, ask whether the target was sensible.

Could the runner actually sustain something close to that demand?

Did they start faster than planned?

Did the opening part of the race cost more than expected?

There is little value trying to improve someone’s durability at a pace they were never fit enough to sustain in the first place.

3. Look for what made the problem happen sooner

This is where pacing, carbohydrate, heat, terrain and hydration come in.

They can change how quickly performance-related qualities deteriorate.

A runner who starts aggressively may expose a weakness much earlier.

A poorly fuelled marathon may turn manageable fatigue into a major collapse.

A hot day may create far more cardiovascular strain than the runner experienced in training.

4. Decide what needs to hold up better

For one runner it could be running economy.

For another it may be muscular force production.

For a trail runner it might genuinely involve eccentric resilience from prolonged descending.

For someone else it may simply be the ability to tolerate sustained event-specific running for longer.

This is the coaching principle I would use:

Training for durability means identifying what is most likely to deteriorate for you, in your event, and preparing that quality to hold up better under fatigue.

5. Create the right exposure

Training then becomes more specific.

That might involve:

• Progressively longer runs.

• Race-pace work later within a long run.

• Greater overall training consistency.

• Appropriate strength work.

• Fuelling practice.

• Terrain that matches the event.

For a hilly race, it may include appropriate downhill exposure.

But none of these should automatically be prescribed simply because somebody “needs more durability”.

The training should match the actual problem.

Training tired is not the goal

If performance deteriorates under fatigue, the obvious conclusion can be:

“I need to do more training when exhausted.”

Not necessarily.

Fatigue may be part of the stimulus, but fatigue itself is not the goal.

You do not improve durability simply by making yourself more tired.

I am looking for the smallest useful training exposure that prepares the runner for the relevant demand while still allowing them to recover and adapt.

That might mean gradually extending a long run.

It might mean a controlled section at race effort later in a run.

It might mean improving strength, practising fuelling or simply building a more consistent training history.

The aim is not to prove how badly you can suffer at the end of training.

The aim is to arrive at race day better able to preserve what you need.

Think about what changes first

Suppose you can hit your target pace when relatively fresh but repeatedly struggle to maintain it as an event continues.

Do not immediately assume you need more speed.

And do not immediately assume you need more mileage.

Ask:

What starts to change first?

Is the pace unrealistic?

Are you spending too much energy early?

Does fuelling become a problem?

Do your legs lose the ability to produce the same force?

Does a particular terrain repeatedly expose you?

Does the problem only appear once you move beyond a duration you rarely train for?

Those answers give you somewhere useful to start.

That is more helpful than simply saying:

“I need to get fitter.”

Because you may already be fit enough to produce the pace.

The problem may be preserving enough of that performance capacity for long enough.

Coach’s Decision

If you can hit the pace fresh but repeatedly cannot hold it, don’t automatically add more speed or more miles.

First ask:

• Is the pace genuinely sustainable for the distance?

• What starts to change as fatigue builds?

• Are pacing, fuelling, heat or terrain making the deterioration happen sooner?

• What specific quality appears to need to hold up better?

Then prepare that quality under conditions that make sense for your event.

Being able to produce the pace is the first test.

Racing well depends on preserving enough of what allows you to produce it.