80m of exercise is enough to deplete muscle glycogen to as-near-zero as makes no difference.
In a sessile starvation state glycogen stops being a meaningful energy source after 2 days, with the majority of it depleted in approximately 8-12 hrs (it's non-linear).
Depending on one's baseline physical activity and what they mean by "half a day," sure, glycogen stores can be reasonably depleted.
That said, glucogenic amino acids can replenish glucose stores, which can ultimately become glycogen. Glycogen remains low only in the context of low-carb and low-total calories, so what glycogen is replenished is soon exhausted.
In a sessile starvation state glycogen stops being a meaningful energy source after 2 days, with the majority of it depleted in approximately 8-12 hrs (it's non-linear).
Depending on one's baseline physical activity and what they mean by "half a day," sure, glycogen stores can be reasonably depleted.
That said, glucogenic amino acids can replenish glucose stores, which can ultimately become glycogen. Glycogen remains low only in the context of low-carb and low-total calories, so what glycogen is replenished is soon exhausted.