Your target heart rate is a very important component when determining your fitness goals. It is approximately 50-80 percent of the maximum that your heart can beat! This "maximum heart rate" or MaxHR, is roughly calculated as 220 minus your age. To get the most cardiovascular benefit , this is the heart rate that you should strive for when working out, after completing your warm-up activities!
To check that you're within your range, you would be required to periodically stop and check your pulse. If you don't want to interrupt your workout, you can use conversation as your measuring tool.
If you can talk comfortably, you aren't working hard enough, but if you get out of breath quickly, and have to stop and catch your breath, then you are probably working too hard.
Beginners should strive to work at 50% of their THR for the first several weeks. A gradual build up to 75% after about six months would be appropriate.
To simply "stay in shape" it is not necessary to work that hard. The most up-to-date research indicates that for "basic health", you should exercise for a minimum of 30 minutes a day, at a moderate level of intensity, 5 days a week. For more significant health and fitness benefits, you would want to double that time or work out at a greater intensity.
This research further states that if you are exercising at an intensity that makes conversation difficult, you can cut the recommended time in half (15 minutes) for basic health.
Research still supports that 10-minute chunks are effective if you can't carve out 30 minutes at one time.
So, determine your target heart rate today and use that information to establish your own training goals! Enjoy!