Exhaustion is in people's heads to an extent, but I think you're lying about your energy intake and/or activity level. If you do three 36 hr fasts per week then you're effectively only eating 2.5 days per week. Consuming 1500 kcal/day on non-fasting days equates to only 3750 kcal/wk total. Running burns at least 90 kcal/mi even for a small person so if you're doing 35 mi/week that means >3000 kcal/wk just from exercise, plus at least 7000 kcal/wk for basal metabolism. At that rate you would therefore be losing about 2 lb/wk of fat, which is an unsustainable for more than a few weeks at a time.
As a point of comparison I'm a large man and fairly active with endurance sports so I have to consume about 3100 kcal/day (with no fasting) to maintain body weight.
Every time the New York Times publishes a health-related article, the comment section is filled with comments just like this: just self-congratulations without any useful takeaways for anyone else, and zero self-awareness that would enable them to realize, hey, not everyone is like me.
Whenever one feels tempted to utter “it’s all in your head”, my advice is to take a step back and reflect on how little I know about the circumstances of others, and then, you know, maybe just keep it to myself.
Behavioural patterns are heavily influenced by hormonal balance and as such, success-rates of different self-help strategies (diets, fasting, resistance and/or endurance training) are highly individual. This also extends to addictive behaviours.
"Hormon-typical" individuals have an easier time shaping their behavior because they don't face imbalances that complicate adherence. For them, sticking to a program is trivial. Combine that with lack of reflection, and many of these individuals delude themselves into thinking their success of following simple programs (which are simple in design, and only difficult in adherence) is somehow an accomplishment worthy of note. Low-empathy individuals, in particular, often interpret this as evidence of their own superiority, while dismissing others as mediocre.
So you see such comments a lot, because many people are "hormon-typical" and also low empathy. See any discussion about diet, fitness, Ozempic, etc.
Sounds like anorexia behavior. Those people do sport a lot, eat a little, right up the the moment the cumulative body damage is just too much. Pro-ana forums are full of people who live on very little calories and are at complete denial about health harm they cause to themselves.
