A. This question has been the topic of much debate throughout the years. The general consensus seems to be... If you want general conditioning and to burn bodyfat, you should perform cardiovascular activities between 65-75% of your Maximum Heart Rate (MHR) for a longer duration. If you are looking to improve cardiovascular endurance and efficiency, your workload should be between 75-85% your MHR for a shorter duration.
My take on it, If you perform a slow activity like riding a stationary bike or walking on a treadmill at a slower pace for about 30-60 minutes, you will burn calories and use bodyfat stores as fuel while sparing muscle breakdown. If you perform an activity such as running, spin class etc at a higher percent of MHR, you will burn a lot more calories in a shorter period of time which in turn creates a caloric deficit and thus bodyfat is lost as well.
It basically comes down to what you are training for. If you are training for an endurance activity or sport, I would recommend training between 75-85% MHR. For general conditioning and fat loss go for 65-75% MHR.
If you fear you're still training wrong, TRY INTERVAL TRAINING! Interval training is when you elevate your heart rate to a higher threshold for a period of time, then back it down to a lower heart rate. For example, run for 2 minutes followed by walking for 1 minute. You can adjust the high and low intervals to any heart rate and duration you like. This is very effective for people wanting to start long distance running. Simply run for a few minutes followed by walking to catch your breath. Eventually the rest or lower heart rate zone time decrease and the higher MHR zone will be a longer duration as you become more conditioned. Try this during your next workout to shake things up instead of cruising along with your cardio mindlessly.