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.