In general while running, you should not always be pushing yourself to the absolute maximum of your fitness/cardio-respiratory capability. If you're running to lose weight or even to stay in good shape (heart heath), a good test is called the "Talk Test".

If while running you can still talk and carry on a conversation with a little difficulty (breathing), you are running at a good pace for you. If you are running so hard that you have great difficulty speaking, you should probably slow  down a little bit.  On the flip side, if carrying on a conversation is really easy, push yourself and pick up the pace a little bit more.

To lose weight or stay in good shape, you should be exercising at a pace where you are pushing yourself.  The Talk Test is a good way to check if you're giving yourself an appropriate aerobic workout. Try it the next time you go on a run with a friend, while on a treadmill, or even during an aerobics class workout.