Aerobic is a cardio workout. Cardio must meet 3 main elements, namely sustainability, rhythm, and aerobic. For maximum cardio, you must challenge your own body by changing the intensity of your aerobic work. Joining the same aerobic class continuously will not benefit you, because your body gets used to it. Staying in your comfort zone means you're wasting your time. In aerobics, no matter how easy the instructor brings his class, you have to get out of that comfort zone, if you want to get results. If you get the desired result (reach the level of maintenance), then you do not have to bother thinking about the intensity. Most people I see, have not reached the level of maintenance, therefore hard work is a must.
Aerobic intensity can be measured in 3 ways, namely the heartbeat, test talk, and ask yourself. Heartbeat is the most accurate way - place your two middle fingers in your wrist artery, or in your neck near the jawbone, where you can count your blood vessels per minute. Test talk means, ideally you can chat without having to breath-breath while undergoing aerobics. Ask yourself, how hard is that aerobic you feel at that moment?