Target Heart Rate Zone Table
To know whether you are improving your heart and lung fitness as you become more physically active, track your target heart rate during your activity. Your target heart rate is a percentage of your maximum heart rate, which is the fastest your heart can beat, based on your age.

Unless you are in excellent physical condition, any physical activity that boosts your heart rate above 75 percent of your maximum rate is likely to be too strenuous. Any activity that increases your heart rate to less than 50 percent of your maximum rate gives your heart and lungs too little conditioning.

To find your target heart rate zone, look for the age closest to yours in the table below. The figures in the table are averages, so use them as general guidelines.

The most healthful activity level uses 50 to 75 percent of your maximum heart rate. During the first few months of your activity program, aim to reach 50 percent of your maximum rate. As you get into better shape, you can slowly build up to 75 percent.
Keep in mind that "getting into the zone" doesn't mean pushing yourself to the limit. For example, while walking briskly or jogging, you should be able to keep up a conversation without trouble. If you can't, move a bit more slowly. Overall, a gentle, gradual approach will help you to maximize your gains and minimize your risks.

Taking Your Pulse
To find out whether you are within your target heart rate zone, take your pulse immediately after finishing your activity.

• As soon as you stop your activity, place the tips of your first two fingers lightly over one of the two blood vessels on your neck, located to the left and right of your Adam's apple. Another convenient pulse spot is on the inside of your wrist, just below the base of your thumb.
• Count your pulse for 10 seconds and multiply by six.
• If your pulse falls within your target zone, your activity is providing good benefits for your heart and lungs. If you are below your target heart rate zone, move just a bit faster next time, as long as you continue to feel reasonably comfortable doing so. If you are above your target zone, move a little more slowly. Don't try to move at, or very close to, your maximum heart rate - that's working too hard.
• Eventually, you will be consistently engaging in your activity within your target zone. To continue to track your progress, keep checking your pulse after at least one activity session per week.

About Medications
A few high blood pressure medicines lower the maximum heart rate and thus the target zone rate. If you are taking high blood pressure medications, contact your doctor to report your target heart rate zone and to find out whether your activity program needs to be adjusted.

Source: http://www.nhlbi.nih.gov/health

What's wrong with pushing yourself to the limit? Anything? I feel like I get a better workout in if I can't keep up a conversation. If I can, on the other hand, after I am done working out I feel like I didn't push myself hard enough and get that much accomplished.