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In Zone 2, your body becomes efficient at utilizing fat as an energy source, tapping into its large fat reserves of over 100,000 calories stored in body fat and about 2,000 calories of glycogen in muscles and liver. Training in this zone teaches your body to rely less on glycogen, preventing premature fatigue. If you regularly operate above this zone, you'll burn through glycogen stores quickly, leading to hitting the proverbial wall.
Finding Your Zone 2
To keep it simple:
If you can hold a conversation in full sentences and breathe through your nose while moving, you're likely in Zone 2.
Alternatively, use heart rate estimations: subtract your age from 180; your upper limit for easy endurance should not exceed this number.