Foundry Files

Foundry Files No. 002: Aerobic Is a Skill

By Coach Sean Fenoff · · 3 min read

By March, most endurance athletes are “fit enough” to start getting themselves into trouble.

The engine is turning over. Workouts don’t feel shocking anymore. Paces are creeping down.

And this is usually where aerobic development quietly stalls.

Not because athletes aren’t working hard, but because they don’t understand what they’re actually trying to build and how to build it.

What “Aerobic” Actually Means

Aerobic training isn’t just “low intensity” or Zone 2.

Below is one of my favorite diagrams, depicting the two most important metabolic foundations in endurance training: Aerobic Threshold (LT1) and Anaerobic Threshold (LT2). Building your aerobic engine doesn’t mean JUST working out around your aerobic threshold… but it does mean spending a decent amount of your training there.

Diagram of aerobic threshold (LT1) and anaerobic threshold (LT2)

It’s work performed below your first metabolic turn point (referred to as AeT/LT1 above), where energy demand is primarily met through oxidative fat metabolism.

In practical terms, that means:

  • Blood lactate production ≈ lactate clearance

  • Fat oxidation is high

  • Stress hormones are controlled

  • Mechanical durability can accumulate

When done correctly over time, this drives:

  • Increased mitochondrial density

  • Capillary proliferation

  • Improved stroke volume

  • Greater fat utilization at higher outputs

  • Lower relative cost at race pace

That’s not fluff. That’s aerobic infrastructure.

You’re not just “getting miles in.” You’re expanding the size of the engine.

Where Athletes Go Wrong

Most athletes drift above true aerobic intensity more often than they realize.

Heart rate creeps. Watts drift. Long runs become steady-state efforts.

The problem isn’t occasional intensity. In fact, no matter what time of the season or phase you’re in, you should have intensity in your program over your aerobic threshold. The problem is chronic gray-zone work, the metabolic dead-zone between your aerobic and anaerobic threshold.

When lactate begins to accumulate faster than it clears, the session shifts from aerobic development to threshold strain.

You feel like you’re working.
You feel productive.
But you’re no longer maximizing mitochondrial signaling or long-term durability.

You’re just accumulating fatigue.

Aerobic development requires metabolic discipline. And discipline is a skill.

Aerobic Efficiency Is Trainable

For instance, let’s say there are two athletes who can run 7:30 pace:

  • One is at 78% of max heart rate.

  • The other is at 88%.

They look the same on Strava.
They are not the same physiologically. Now, heart rate isn’t everything, but it is important.

The more aerobic athlete has:

  • Greater stroke volume

  • Better peripheral oxygen extraction

  • Lower sympathetic activation

  • More metabolic flexibility

That gap isn’t talent. It’s years of correctly dosed aerobic work.

The real goal of March isn’t to run faster. It’s to lower the cost of running at any speed.

Efficiency first. Speed later.

Why This Phase Precedes Performance

You cannot shortcut oxidative capacity.

Anaerobic threshold work without aerobic depth leads to:

  • Early-season sharpness

  • Mid-season stagnation

  • Late-season breakdown

The aerobic system supports everything:

  • Lactate clearance during surges, hills, etc.

  • Recovery between intervals

  • Durability deep into long races

  • Heat tolerance

  • Glycogen sparing

If February was about purpose, March is about physiology.

Build the base wide enough, and everything above it stabilizes.

Rush it, and everything above is unstable.

From the Foundry Floor

When I review my athlete training files, I look at decoupling. What do I mean by this? I mean:

  • Does heart rate drift upward at the same pace?

  • Does power drop while effort rises?

  • Does form deteriorate late in steady sessions?

Aerobic skill shows up as stability. Constant over duration.

Low cardiac drift. Stable power. Even splits. Controlled effort and breathing.

The athletes who master this don’t look impressive in March, well because, it’s not glamorous by any means. It’s humble, constrained work.

But, they look inevitable in September.

Closing Thought

If you want a simple test, during your next longer session, ask yourself:

  • Can I nasal breathe comfortably?

  • Is my heart rate stable after 40–60 minutes, or is it slowly rising?

  • Could I extend this effort another 20–30 minutes if needed?

If the answer is no, you’re not undertrained. You’re underdeveloped aerobically.

And that’s not a criticism. It’s an opportunity.

Aerobic capacity doesn’t respond to hype. It responds to precision and consistency.

— Sean
Foundry Athletics Co.