While maintaining this pace for 20-30 minutes will render benefits in weight loss and conditioning and is a good course for older participants or those with restrictions, the fact is the caloric expenditure is too small.
First off let me say, always have a check up with a physician before undertaking a vigorous exercise program. That being said, you are better served burning the maximum amount of calories youre capable of in the time allowed. A calorie burned is a calorie not on you. Moderate and especially high intensity exercise causes the majority of calories to come from carbohydrates in the form of glucose or glycogen found in your muscle, blood and liver. Glycogen with the usage of oxygen (hence aerobic exercise) converts easily to a usable form of energy to power you. The truth is your system will continue to burn some fat for fuel as well as glucose in an effort to spare carbohydrate for other body functions. The real benefit comes from the fact that all this expended sugar needs to be replaced meaning more of the carbohydrates you eat will go toward restoring depleted glycogen stores and less of it stored as fat.
If fat loss or fitness conditioning are your main, goal try interval training. Start with a 5 10 minute warm-up then as you proceed vary your pace between a walk-run, jog-run, or run-sprint format. Try hills and flats for variety. As you improve you can extend the interval periods of high intensity. Not only will you burn more fat and overall calories during your workout, but you increase your after burn as well over conventional training methods. Be sure to vary your routines often to prevent stagnation.
To check your heart rate place your index and middle finger next to your windpipe and push gently up and in to feel pulse. Hold for 10 seconds counting pulses and multiply by 10 to get heart rate. Holding for longer or repeatedly checking every few minutes increases error. There are also many excellent inexpensive heart rate monitors available in sporting goods stores. In theory your maximum heart rate is the number 220 minus your age for men and 226 minus your age for women. Below 65% times this number is considered light intensity and above 85% is heavy.