Q&A #44 - Carb/fat combustion based on pace and heart rate; VO2max intervals on flats or uphill

If heart rate was in zone 4 and pace in zone 2: did you use mostly carbs or fats for fuel?
Can you do track/road intervals on hilly trails, and if so, should they still keep the same structure or can they be free form?
Should mobility work count towards your total training duration?

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