The 12th and 13th weeks together outlined a clear process that made it visible what fat-based metabolism actually looks like in action — and where the real limits lie for an ultrarunner.
Week 12 was a classic accumulated fatigue block. Running sessions and kettlebell OALC (One Arm Long Cycle) loads reinforced each other, creating a complex stimulus for the body. This wasn’t just about building endurance; it continuously shaped mechanical and nervous system adaptation as well. In weeks like this, it’s not individual standout workouts that matter most, but whether the body can sustain its function day after day without falling apart — either energetically or neurologically.
It was in this environment that what I would clearly describe, based on my own experience, as a metabolic shift occurred. During runs, the pace became more stable, heart rate responded less dramatically to effort, and — perhaps most importantly — the inner uncertainty that had always appeared on longer distances disappeared. Two personal bests were set during this week, not as the result of one exceptional day, but as a sign that the entire system had begun operating on a new level.
For me, one of the most important signs of metabolic adaptation was not visible in the measured numbers, but in the quality of functioning. The pace remained even, heart rate didn’t drift upward, there was no breakdown at the end of runs, and crucially, the classic energy crash that had often limited performance in the past did not appear. This perfectly matches the observation that true adaptation is not shown by a single biomarker, but by the overall stability of performance.
By the end of the week, however, a subtle but clear signal also appeared: sleep quality declined. Not dramatically, but noticeably. For me, this was not a problem, but feedback that the body was trying to adapt to a higher training load. Within a few days it resolved, and recovery returned — once again confirming that the system was capable of processing this level of stress.
Week 13 revealed another aspect of this process, this time in a competitive setting.
On Saturday came the Hungarian Kettlebell National Championships — OALC half marathon with a 16 kg kettlebell in the under-74 kg weight category. The result was 353 repetitions and 1st place. This performance was not a sudden breakthrough, but the consequence of the system remaining consistently stable in the preceding weeks and being able to keep the load under control. During the long sets, the question was not “how long can I last?”, but whether I could maintain the same rhythm and technical quality all the way through.
The next day was the 100 km run. The weather was far from ideal — cold, windy, and rainy — yet the conditions did not meaningfully affect my functioning. The pace remained stable, energy supply stayed consistent, and for more than 30 kilometers there was no fatigue or energy drop. The run showed exactly the picture that Week 12 had already foreshadowed: the system works.
And yet, this is where the process hit a clear limit.
At one point, my left ankle became unstable, and shortly afterward it reached a state where I could no longer put weight on my foot. This wasn’t simple pain — the leg literally collapsed under load and could not support any weight. I tried to restart several times, but the same thing happened every time: as soon as weight was applied, the ankle “shut down” and prevented movement.
After changing shoes and socks, the pain decreased significantly. This clearly indicated that the issue was not general exhaustion, but a local, mechanical problem that only appeared under load.
This was the moment when the decision had to be made.
I didn’t stop because I ran out of energy or because the pace was no longer sustainable. I stopped because the basic prerequisite of movement had disappeared. The energetic side of the system was still functioning, but the mechanical side no longer allowed continuation.
One of the most important lessons of the week comes precisely from this:
During prolonged efforts, it is often not energy supply that represents the real limit, but the mechanical resilience of the body. The cardiovascular system and metabolism would be capable of sustaining the effort, but at a certain point the cumulative stress from repetitive movement builds up, and the weakest link in the system becomes the limiting factor.
These two weeks together thus painted a very clear picture:
Fat-based metabolism provides stable energy supply, reduces fluctuations, and allows performance to remain even for a long time. Once this stability is established, it also becomes visible that the true limits of performance are not set by the energy system, but by the mechanical structures.
It is not energy that runs out first.
It is the structure.