Running at zone 2 (60-70% of max heart rate) does build real aerobic endurance and lays groundwork for faster races — but framing it as the specifically superior route to "mitochondrial density" goes further than a recent critical review confirms.

noli's read

Stripped of its educational packaging (emojis, runner myths framing), the claim tracks the physiology of endurance training reasonably well: at 60-70% of max heart rate, the body favors fat oxidation as fuel, and training at this intensity is documented to improve cardiac stroke volume (the heart pumps more blood per beat) — a mechanism consistent with the easy-running literature. The claim that this easy-running volume lays groundwork for faster 5K and 10K times is consistent with the aerobic-base training model widely used by coaches. However, framing zone 2 as THE way to specifically and superiorly "expand mitochondrial density" is challenged by a recent narrative review (Storoschuk et al. 2025), which finds the evidence for zone 2's superiority on this point limited and mixed against higher-intensity training at matched volume. The general principle (don't skip easy mileage) holds; the precise mechanistic framing around mitochondria is more uncertain than the post's confident tone suggests.

the sources

1. 1Rosenblat MA, Perrotta AS, Vicenzino B 2019
2. 2Silva Oliveira P, Boppre G, Fonseca H 2024
3. 3Menshikova EV, Ritov VB, Ferrell RE, et al. 2007