The Run 'Talk Test'

In general when you run, you do not always want to be pushing yourself to the absolute maximum of your fitness/cardiorespiratory ability.  If you're trying to run to stay in good shape and have god heart health, a good test is called the 'Talk Test'.  If while running you can still talk and carry on a conversation with little difficulty, you are running at a good pace.  If you are running so hard that you have great difficulty speaking, maybe tone it down a little bit; and if carrying on a conversation is easy, push yourself a little bit more.  Again, you should be exercising so that you are pushing yourself, and the Talk Test is a good way to check if you're giving yourself a good aerobic workout.  Try it the next time you go on a run with a friend, or even during a workout.