Good point raised.
Actually i feel these topics in nutrition/recovery are not often discussed enough....... recently (after CNY period) i feel the recent boost was due to just changes and tweaks to my recovery....be it big or small. My so-called training did not change at all and is more or less what i can do wrt my lifestyle schdule. Any nutritional deficiency, from carbs to proteins to micronutrients, can really hamper recovery, which is basically the other 50% of the equation.
eg your said iron....the incidence of some sort of iron deficiency in women is as high as 50% doing marathon training with high mileage.
Especially so that you all running long distance, must take lah.....esp if not really paying attention to diet. Esp zhar boh which is higher risk than men, or if 100% eating out and predominantly hawker food. Unless if taking sufficient meat + liver (eg cai png). For me, i am taking 2-3x a week 10mg (very safe), in addition to diet food, in which i'd take at least 2 times a week liver from cai png.
But i get it, some dont like pork liver from cai png due to smell, and that is a legit thingy. Luckily my cai png store cooks it pretty decent esp my 11.30 lunch time when it is still pretty fresh from the wok as they usually cook it at 10-10.30am.
For shorter distances not so terok. For 3k 5k i think can chill, largely non-issue as far as chronic deficiency is concerned.
If one is really afraid of "anyhow taking supplements" esp since iron is not a totally benign supplement, just check FBC even if dont wanna pay for iron panel, more or less can get the story liao even if FBC is not definitively diagnostic by itself. Just talk to your GP.
But of coz, if one's Hb is in the single digits on your full body checkup test, then no need simi iron panel ordered by a specialist, "Houston, we REALLY have a problem".

Anyway the GP consulation in the body checkout test also def would flag this out, unless if you skip the consulation after results come out.
Other than sleep (recovery), any of these these deficiencies and similar hampers recovery and performance adaptations, which is just as important as the training itself
edit - this article mentioned as high as 56%

for those really into some sort of competitive running, maybe they mean with some decent goals and not suka suka runners.
https://runnersconnect.net/iron-deficiency-in-runners/