5 Biomarkers to Check Before Running a Marathon: Are You Really Ready?

Every year, runners come to me with their bib number already assigned, the 16-week plan already printed, and a question they ask almost in passing: "Do you think I'm ready?" The right question isn't how many miles you've logged over the last twelve weeks. It's what's happening inside your body while you're logging them.

Volume is visible. Biomarkers aren't, and that's exactly why they get ignored. But they're the ones that actually determine whether you show up on race morning with an engine that can sustain 3 or 4 hours of effort, or with a body that's already asking for mercy before mile 18.

Here are five I check, in order of priority, before giving a marathon goal the green light.

1. Heart Rate Variability (HRV)

HRV measures the time between heartbeats, and that pattern shifts based on how much stress — physical and psychological — your autonomic nervous system is absorbing. It's not magic, it's R-R interval math.

What matters to me as a coach isn't a single day's number. It's the 7-to-14-day trend. HRV that drops in a sustained way, paired with an elevated resting heart rate, is the earliest sign that training load is outpacing recovery capacity. If that pattern shows up during the highest-volume weeks of a marathon block — right when the athlete wants to "ride the momentum" — that is precisely the worst week to add more.

What to do: Measure at the same time of day, in the same position (lying down, upon waking), for at least two weeks before committing to your volume peak. If the trend drops for three or more consecutive days, that is not the week to schedule the longest run of the plan.

2. Ferritin and Serum Iron

This is the one that surprises me most in runners who "feel fine" but underperform relative to their training. Low ferritin — even without reaching clinical anemia — reduces oxygen-carrying capacity and directly affects mitochondrial metabolism, because iron is a cofactor for several enzymes in the respiratory chain.

In distance runners — especially premenopausal women and athletes training high volume in heat or altitude — iron loss through sweat, urine, and small gastrointestinal bleeds during long runs is real and cumulative.

What to do: Get a full panel — ferritin, serum iron, transferrin saturation — at least 8 weeks before your hardest block, not the week before the race. Ferritin corrects over weeks, not days. A value below 30 ng/mL in a competitive runner already warrants nutritional intervention, with or without anemia depending on hemoglobin.

3. Testosterone and Hormonal Profile

This marker gets little airtime in recreational running, but it's central for masters athletes — men and women — training high, sustained volumes for months at a time. Low testosterone has been linked not only to reduced muscle recovery capacity, but to a complex, multifactorial relationship with vascular and metabolic health, including higher cardiovascular risk when it coexists with excess body fat [PMID:26590935]. That bidirectional relationship — testosterone deficiency worsens the metabolic profile, and excess visceral fat worsens testosterone — is exactly the kind of loop a masters marathoner doesn't want to trigger unknowingly.

In my experience with masters athletes (I compete on the track myself, 45-49 age group), excessive training without adequate recovery is one of the most common ways to push that hormonal profile downward, especially during marathon blocks with double sessions and too little sleep.

What to do: If you're a masters runner dealing with persistent fatigue, muscle mass loss despite training, or reduced libido alongside underperformance, a basic hormonal panel is worth doing before committing to a high-volume plan. It's not for everyone, but it's cheap insurance compared to losing an entire block to undiagnosed overtraining.

4. Body Composition and Fat % (Not Just Body Weight)

The number on the scale tells you very little. What matters to me is the ratio of fat mass to lean mass, and how it shifts based on the type of training you're doing. A recent study comparing cycling HIIT versus running HIIT in men with overweight/obesity found comparable fat loss between the two modalities, but with differences in gut microbiota composition depending on exercise type [PMID:38233990]. This is relevant to marathoners in two ways: first, it confirms that cycling-based HIIT can be a valid body recomposition tool without the joint impact of running, useful for athletes with an injury history who need to shed fat before adding running volume. Second, it suggests that gut health — key to gel and carb tolerance on race day — responds differently depending on how you train, not just how much.

What to do: If you're heading into a marathon block carrying excess body fat, consider 4-6 weeks of cross-training (cycling, low-impact HIIT) before ramping up running volume. That's not sacrificing sport-specific training time — it's protecting your joints while improving body composition, and possibly, along the way, your gastrointestinal tolerance on race day.

5. Sleep Quality and Quantity

Technically not a "blood marker," but it functions with the same logic: it's a quantifiable signal that predicts whether your body can absorb the load you're asking of it. Sleep is where most muscle protein synthesis happens, where cardiovascular training adaptations get consolidated, and where cortisol regulation takes place.

Runners who consistently sleep less than 6.5-7 hours during a marathon block show, in my experience with TrainingPeaks athletes, lower HRV patterns, higher perceived effort at the same pace, and more overuse injuries. The relationship between sleep and HRV (marker #1) isn't a coincidence — both are windows into the same autonomic nervous system.

What to do: Use a wearable, or simply a sleep log, for two weeks before committing to the peak of your plan. If you're averaging under 6.5 hours, that's the first problem to solve — before touching anything in the training plan itself.

The Coach's Perspective

None of these five markers, on its own, tells you "don't run the marathon." What they tell you, together, is whether your body is in shape to absorb the next 12-16 weeks of load without breaking down before the start line. I review them as a panel, not as individual stoplights — low HRV with good sleep and normal ferritin is a very different story from low HRV paired with ferritin at 18 ng/mL and 5.5 hours of sleep.

The blunt honesty I owe my athletes is this: the perfect training plan does not compensate for a body that isn't ready to receive it. Checking these markers doesn't delay your marathon — it protects it.

Key Takeaways

• HRV: Track the 1-2 week trend before ramping up volume, not a single day's reading.
• Ferritin/iron: Get the panel 8 weeks before your training peak, not race week.
• Testosterone: Especially relevant for masters athletes with persistent fatigue and muscle loss despite training.
• Body composition: Consider low-impact cross-training before increasing running volume if you're carrying excess fat.
• Sleep: Aim for a consistent 7+ hours; it's the foundation the other four markers rest on.

References

• Couvert A, Goumy L, et al. Effects of a Cycling versus Running HIIT Program on Fat Mass Loss and Gut Microbiota Composition in Men with Overweight/Obesity. Med Sci Sports Exerc. 2024. [PMID:38233990]
• Traish AM, Zitzmann M. The complex and multifactorial relationship between testosterone deficiency (TD), obesity and vascular disease. Rev Endocr Metab Disord. 2015. [PMID:26590935]