Depends if you want fitness or weightloss.

If you want weightloss you need to keep your heartrate low enough that your body is still burning fats. Once you raise it when running etc, you begin to burn glycogen and carbs therefore not burning fats.

I can't remember the exact % of max heartrate but i am pretty sure that it needs to be
below 65% to burn fat
below 80% for fitness (aerobic exercise)
then 80-90% is to improve your anaerobic system (<1k races)
90-95% is for ATP system (sprints)
& to work out max HR is 220-your age