To get the most benefit from your workouts, the American Heart Association recommends staying within your target heart rate range. Your target heart rate is your resting heart rate plus 50 – 85%. The Mayo Clinic web site has a quick target heart rate calculator you can use.
Of course, measuring your target heart rate can be a little complicated. The AHA also recommends the “conversation pace” measurement. If you can talk while you workout, you’re probably going at a good pace. If you can sing while working out, you need to push yourself a little harder. If you cannot talk without a lot of effort, you could be working out too hard.