Not Sick, Just Stupid: The Hidden Cost of Tempo Runs (Post Marathon Low Carb Flu)

May 16, 2025 | By DLake

A few weeks ago (2.5 weeks after my marathon), I did a Rosario 800 float workout. I felt amazing during it, like that “I’m so back” feeling. I even biked to my meeting after, uphill, with all of my gear. The classic “I’m fit again” energy. I even made sure to fuel up like crazy, hydrate, electrolytes, etc., because it was a bigger morning.

But by 4 pm, I got weird body chills, I felt weak, and my body was like, “nah man, we’re done here.”

Not sick. No fever. No sore throat. Just straight-up glycogen stores depleted, aka central nervous system fatigue, aka Post Marathon Low Carb Flu. Classic mistake: I ate “well” but not “enough” and not straight carbs. I slid too quickly back into my low to moderate carb intake because I don’t like eating a lot of carbs. Too many carbs make me feel weird.

Let’s rewind for a second

Even though I was almost 20 days post-marathon, I was still low on carbs/glycogen. The tempo run didn’t drain me — it was the three days before that did the damage.

Sunday, I did my first long run in two weeks. Ate normal, but still running on empty from the marathon. Didn’t refuel enough. Later that day, I hit a strength session because I felt fine. More carb depletion.

The next day, I wanted to be productive — wake up early, get stuff done. So, I fasted like usual in the morning, didn’t work out until 12 pm, trying to power through. When I train later, I don’t eat much beforehand. Finally ate after my 1 pm home strength workout, but overall calories were way too low. A few hours later, I went for a short, easy run with some strides and skips.

Right after, I was starving. Like “haven’t eaten in day,s” starving. I ate all night. Next morning, I woke up and went straight into my Rosario 800s workout. Took in some carb electrolyte mix before, but it wasn’t enough — not after the beating my body took those 2-3 days before.

Tempo floats are sneaky, and they don’t spike HR crazy high, so you feel like it’s not that hard, but they drain the tank slowly till you’re running on fumes.

Fast forward a few days later and I woke up better but still had those annoying chills and that “fragile” vibe going on. I ignored my ego and kept the run easy. I drank water every 5 mins, smashed gels like it was a race. HR was low, felt smooth after 15 mins. I still wasn’t 100%, but definitely feeling better. It took me another few days to get rid of the chills feeling on my arms. So odd.

The lesson learned? This wasn’t a sickness thing, but it was an under-recovery thing. Mybody’s still patching itself up post-marathon, and even though I was 2.5 weeks out, it still wasn’t enough. The marathon takes 3-4 weeks to recover from properly, and that’s building up the carb/glycogen stores again.

Tempo days + life stress = glycogen debt and I can’t out-hustle physiology… yet (maybe in the next 5-10 years with AI or something).

Ibuprofen got me through some work days, so I wasn’t feeling terrible, but it’s like duct-taping the oil light on your car. It doesn’t fix the problem. You need carbs, salt, sleep—not a painkiller.

So yeah, reminder to Future Me (and you):

After a big session, especially post-marathon, eat until you’re slightly too full, salt your food, and prioritize boring recovery.