No worries and I should have been more descriptive. But since the context was within aerobic threshold and endurance, I was referring to the amount of calories

No worries and I should have been more descriptive. But since the context was within aerobic threshold and endurance, I was referring to the amount of calories burned from fat grams, per hour. Elite athletes can recruit energy from over 1g of fat per minute. At 9cal/g, we are looking at around 600 cal/hour. Poorly conditioned individuals (i.e. low thresholds) are less efficient and therefore their bodies resort anaerobic glycolysis much sooner.