That's because the oxygen is no longer being delivered to your muscles. Your anaerobic system is designed only for those short bursts of intense activity. Supermarket shopping, on the other hand, will burn some calories, but it's not rigorous enough to be considered aerobic. There is a very simple way to determine how much aerobic exercise you need. Three to five times per week.
Intensity plays an important role in the FIT formula, and it's the hardest factor to judge.
You can also use the Rating of Perceived Exertion. Use this scale to gauge how hard you're pushing throughout your workout. The talk test is another simple way to gauge your intensity. While performing any aerobic activity, you should be able to carry on a casual conversation without becoming breathless. So, if fat burning is your goal, it seems to make sense to exercise longer at lower levels.