When Your Body Stops Cooperating: A Triathlete’s Mystery at 6 Weeks Out

It didn’t announce itself.

The first run that fell apart, I shrugged off. Everyone has off days. Bad sleep, ate too late, weather was warm, legs were heavy from the bike the day before — there’s always a reason. I logged it, moved on, didn’t think much of it.

Then it happened again. And again. And again.

By the third or fourth time I cut a run short or limped through the back half of one, I started paying attention. By the sixth or seventh, I knew. This wasn’t a string of bad days. This was something. The pace I usually hold easily felt impossible. My legs turned to cement somewhere around the half-hour mark. I hit a wall I couldn’t push through, and no amount of grit was going to break it.

For a triathlete, this is disorienting in a particular way. We’re used to suffering. We’re used to bad days. We know what bonking, cramping, overheating, undertraining, and overtraining feel like. This wasn’t any of those. It was something else, and it kept showing up.

What made it stranger: I could still ride. I could get on the bike and put in a real session and feel almost normal. Swim was fine. But running — specifically running — fell apart somewhere around the 30-minute mark, every single time.

That asymmetry turned out to be the most important clue.

The Running vs. Cycling Puzzle

Here’s something most people, including a lot of athletes, don’t know: running and cycling stress the autonomic nervous system — the part of your nervous system that runs everything you don’t consciously control — in very different ways.

Running is an upright, weight-bearing, orthostatic activity. Your heart has to work against gravity to return blood from your legs back up to your brain. Your autonomic nervous system has to constantly adjust vascular tone, heart rate, and blood pressure on the fly to keep blood flowing where it needs to go — especially to your brain. Every foot strike creates a demand on the muscle pump in your calves to help push blood back up. It’s a coordinated, gravity-defying performance happening below your awareness, and it depends entirely on a well-tuned autonomic system.

If your autonomic system has been knocked off balance by a viral illness — something well-documented after Epstein-Barr (EBV, the virus that causes mono) and a long list of other infections — running exposes it quickly. You notice fatigue out of proportion to effort. The legs feel disproportionately heavy. There’s a wall you can’t push through, no matter how fit you are or how hard you try.

Cycling is a completely different story. You’re seated and supported. Your legs are closer to heart level. Venous return is much easier because you’re not fighting gravity the same way. The rhythmic pedaling acts as a continuous muscle pump, doing some of the work your autonomic system would otherwise have to micromanage. You’re working hard, but the orthostatic challenge — the gravity tax — isn’t there.

This isn’t theoretical. Cycling, especially recumbent cycling, is literally the standard starting point in formal rehab protocols for people recovering from post-viral autonomic issues. Those programs deliberately start patients on bikes and gradually progress over months to upright exercise, precisely because they bypass the orthostatic challenge that breaks people in the early stages of recovery.

So an athlete who feels fine cycling but falls apart running after a viral illness is a fairly classic presentation. Not unique. Not mysterious to those who know what they’re looking at. Just under-recognized in the general medical world, especially in athletes who don’t fit the typical picture of someone who’s “sick.”

The Signal I Missed (And Why I Love Being a Garmin Athlete)

Here’s the part that, in retrospect, I should have paid attention to.

Late February. My HRV — heart rate variability, the most sensitive non-invasive window into autonomic nervous system function — tanked for nine straight days. Not a one-day dip from a hard workout or a bad night’s sleep. Nine days of suppressed HRV, which is exactly the fingerprint of an acute viral hit on the autonomic system. I felt off during that stretch. Took a few couch days. Wrote it off as a minor bug, rested, got back to training, and got on with my life.

And this is exactly why I love being a Garmin athlete and why I wear my watch 24/7. Without that continuous data, that nine-day window would have been a vague memory of “feeling off in February.” Instead, it’s a clearly documented autonomic event with a beginning, a middle, and an end, visible right there in my HRV Status, Body Battery, and training readiness scores. Garmin captured the story my body was telling before I had the words for it.

I didn’t connect it to anything at the time. But that drop is exactly what an acute viral insult looks like on the autonomic system, and the timing lines up uncomfortably well: late February disruption, exercise intolerance emerging in the weeks after. The body was telling the story in real time. I just wasn’t reading it yet.

This is one of the harder lessons of post-viral syndromes — they often don’t show up during the acute illness itself. The acute illness can be mild, even unremarkable. The trouble emerges later, when you try to return to normal training load and discover that something underneath has been quietly destabilized. Having a continuous data record makes the difference between “I think something happened back in February” and “here is exactly when something happened, here is how long it lasted, and here is the trajectory since.” That’s the kind of context a clinician can actually work with.

The Other Clues That Were Already There

A few other pieces of my history that now feel relevant.

I’ve had EBV in the past — like most adults. EBV is one of those viruses that around 90 to 95 percent of us have been exposed to, usually in adolescence or young adulthood, and once you’ve had it, it stays in your body for life in a latent state inside your immune cells. It doesn’t get cleared. It just waits. And under the right conditions — physical stress, another illness, immune load, poor sleep, overtraining — it can reactivate. Reactivation often doesn’t look like classic mono. It looks like fatigue, a lingering sense of being unwell, vague flu-ish feelings, and sometimes exactly the post-viral exercise intolerance picture I’m describing.

I also get cold sores periodically. Cold sores are caused by HSV-1, another herpesvirus that, like EBV, establishes lifelong latency and reactivates under stress. Recurrent cold sores are a sign that my immune system already has a demonstrated pattern of letting latent herpesviruses cycle back to life when it’s under load. The same mechanism that lets HSV-1 reactivate on my lip can let EBV reactivate elsewhere in the body. It’s not proof of anything — but it’s a consistent pattern, and worth noting.

Then, about 10 days ago, I caught a cold. Small thing on paper. But layered onto an already-stressed system, even a minor infection can set things back. People who study post-viral syndromes call this the “second hit,” and it’s a real, documented phenomenon. And again — I could see it on my watch. Resting heart rate up, HRV down, Body Battery struggling to recharge. The numbers told the truth before I wanted to admit it to myself.

The Lab Question

EBV antibody panel. The full panel tells a more nuanced story than the quick monospot test, which is notoriously unreliable, especially weeks out from an actual infection. It measures several antibodies that, together, can distinguish recent infection from past infection from reactivation.

Here’s where it gets tricky for someone like me who’s had EBV in the past: the antibody that marks past infection (EBNA IgG) stays positive for life. So it doesn’t help distinguish “old infection” from “old infection that’s now reactivating.” What matters in my case is whether other markers — specifically early antigen antibodies (EA-D IgG) and the level of viral capsid IgG (VCA IgG) — are elevated. Those would suggest the latent virus has woken up.

If the panel comes back showing only stable past infection without reactivation markers, that doesn’t close the case. It just shifts the focus. Other viruses can produce the same post-viral picture: CMV (cytomegalovirus), HHV-6, parvovirus, even a stealth COVID infection that didn’t announce itself loudly enough to get tested at the time. In a lot of post-viral cases, the specific pathogen is never definitively named. The clinical picture is what drives the diagnosis and the management, not the lab confirmation. Frustrating from a “give me an answer” perspective, but it’s the reality of this corner of medicine.

What matters more than the specific virus is the pattern.  My pattern: an autonomic disturbance in late February visible in HRV, a clinical syndrome of exercise intolerance emerging in the weeks after, a second viral hit ten days ago that may have set things back, and a body of evidence — cold sores, post-EBV history, the running-versus-cycling asymmetry — pointing toward post-viral autonomic dysregulation as the working framework.

The Race

I’m racing in 10 days. I’m racing anyway.

Not to prove anything. Not to push through and risk making things worse. But because I want to see where I actually am — to get real-world data on what my body can and can’t do right now, in race conditions, instead of guessing from training. I’ll go in with realistic expectations, listen to what shows up, and pull back if my body tells me to. The swim and bike should be okay. The run is the unknown.

I’m framing this race as information, not a verdict. Whatever happens on race day is one data point in a longer recovery process, not a referendum on whether I’m “back.” And I’ll have my Garmin on the whole time, capturing every bit of it — heart rate, HRV response, recovery trajectory afterward. The race itself will produce a dataset I can learn from, regardless of the finish time.

What I’m Learning About Recovery

The hardest part of this, for any athlete, is that the instinct that has carried us through every other setback — push harder, train through it, grind it out — is exactly the wrong instinct for post-viral exercise intolerance. The push-crash cycle, where you attempt a hard run, fail, recover for days, then try again, actually entrenches the problem rather than resolving it.

What seems to work instead, based on what’s known about these conditions: protect the non-provocative training (cycling, swimming, easy strength work), stay below the symptom threshold rather than blasting past it, hydrate aggressively and increase salt intake, sleep more than you think you need, and let the running come back gradually as the system rebuilds its capacity. Most people see substantial improvement over weeks to months when they manage it this way. Pushing through tends to stretch the timeline out.

This is another place where wearing my Garmin 24/7 has changed how I navigate things. Training Readiness, Body Battery, HRV Status, and overnight recovery scores give me objective check-ins every morning instead of relying purely on how I feel — which, as any athlete knows, can be misleading in both directions. Some days I feel fine and the data says back off. Some days I feel rough and the data says I’m actually recovering well, just emotionally fried. Trusting the data over the feeling is a skill, and it’s one that matters even more during a recovery process like this.

For a triathlete, there’s almost a built-in advantage here: I have two other disciplines I can train hard in while the run rebuilds. That’s a gift. Most runners don’t have that off-ramp.

What I Want Other Athletes to Know

If you’re an athlete reading this and any of this sounds familiar — the wall that wasn’t there before, the disproportionate fatigue, the run that broke while the bike stayed intact, the string of “off days” that turned out to be something more, the HRV drop you didn’t think much of at the time — you’re not imagining it, you’re not out of shape, and you’re not weak. Post-viral exercise intolerance is real, recognized, and increasingly well-understood, especially in the years since COVID pushed the medical world to take long-tail viral effects more seriously.

A few things I’d offer based on going through this.

Wear your watch. All the time. This is the single biggest reason I’m grateful to be a Garmin athlete — the continuous, 24/7 data turns vague memories into actual evidence. HRV drops, resting heart rate elevations, sleep score crashes, Body Battery patterns — those numbers exist for a reason, and they’re there whether you’re looking at them in the moment or going back six weeks later to figure out when something started. If something’s off for more than a few days, it’s worth taking seriously, even if you feel okay enough to train.

If you have a history of herpesvirus reactivations — cold sores, shingles, recurrent EBV symptoms — know that these viruses can come back to life under training and life stress, and the symptoms aren’t always obvious.

Standard labs being normal doesn’t mean nothing is wrong. It means the standard panels weren’t designed to find what’s wrong. Ask about a full EBV panel, including the reactivation markers, not just a monospot. Ask about CMV. Ask about cardiac clearance — an ECG and an echo — before returning to hard training after any significant viral illness, because myocarditis is the one thing you absolutely don’t want to miss.

And give yourself permission to recover at the pace your body actually needs, not the pace your training plan demands. The fitness will come back. The system will heal. The run will return.

I’ll let you know how the race goes — and the Garmin will, too!