Here is a method to develop your "engine" that is not for the faint at heart or for rookies:
1. Calculate your maximum heart rate
The easiest way to do this is a simple paper-and-pencil calculation. Subtract your age from 220. The result is an age-predicted maximum beats per minute.
This method does not take into account your fitness level or inherited genes, which can make your true maximum heart rate 10 to 20 beats per minute higher or lower than the age-predicted number.
A second method to calculate your maximum heart rate is to have an exercise tolerance or stress test. This usually is supervised by a physician and performed in a hospital or clinical setting in three-minute stages, during which the speed and incline continue to increase in an effort to elevate your heart rate until it climbs to its highest level.
2. Determine your resting heart rate
Take your pulse before you get out of bed in the morning. Do this for several days in a row to get consistent readings.
3. Calculate your heart-rate reserve
Subtract your heart's resting rate from your maximum rate.
For example, if you are 40 years old, subtract that number from 220; your maximum rate is 180. Next, subtract your resting rate, 80 in this example. Your heart-rate reserve is 100 beats per minute.
This heart-rate reserve represents the cushion heartbeats available for exercise.
4. Calculate your aerobic training heart-rate range for fat burning
This fat-burning range will lie between 50 and 75 percent of your heart-rate reserve.
Using the example above, 50 percent of 100 beats per minute is 50. And 75 percent of 100 is 75. Next, add your resting heart rate to both numbers: 50 + 80 = 130 and 75 + 80 = 155. Your heart rate during aerobic training that will most efficiently burn fat is 130 to 155 beats per minute.
5. Calculate your aerobic training heart rate for fitness
The range required to improve aerobic endurance is higher than that needed for fat burning, between 75 and 85 percent of your heart-rate reserve.
Again using the previous example, 75 percent of the heart-rate reserve of 100 is 75, and 85 percent is 85. Again, add the resting heart rate to both numbers.
Re-add your resting heart rate to both numbers: To improve your aerobic endurance, you need to aim for between 155 and 165 heartbeats per minute.
6. Calculate your aerobic-anaerobic threshold heart-rate range
This range represents the upper limits of aerobic exercise -- the point just before you push yourself into exhaustive anaerobic work. Exercising at this intensity is usually done to improve athletic performance. It is not recommended for weight loss.
The range to accomplish this task lies between 85 and 90 percent of your heart-rate reserve. Again, using the example of a person with a heart-rate reserve of 100 and following the same math process as in previous steps: the desired range would be 165 to 170 beats per minute.
Be advised, however, operating at this intensity level will not burn body fat. It becomes a carbohydrate (muscle-glycogen burning) exercise.
7. Calculate your anaerobic training heart-rate range
This is all-out effort and represents 90 to 100 percent of the cushion of your heart-rate reserve. The goal here is to go as fast as you can for as long as you can.
Using the same example, anything from 170 beats per minute to your maximum of 180 beats per minute becomes pure anaerobic, carbohydrate-burning, exhaustive, lactic acid-producing exercise.
This is no-pain, no-gain type training.
from
Based on this, 140 is a little light for you. When I run, I am generally 140 - 155 and this is hard on me.