A) Intensity: The amount of energy or effort expended.

Recommendation: 55% to 85% of your target heart rate.

Target Heart Rate (THR) formula.

Example using 33 as Age:
220-33 = 187 (this is 100% intensity)
187 x .85% = 159 heart beats counted in 1 minute
To find your target heart rate: Use a second hand clock.
Count the # of beats for 15 seconds and multiply by 4.

B) Duration: The amount of time spent during each workout.

20 to 60 minutes at your chosen target heart rate.
Caution: This does not include your warm-up or cool down time unless you are a very beginner.
Example:
Warm-up = 5 minutes
Increase intensity between 60% and 80% = 35 minutes
Cool down = 10 minutes
Total workout = 50 minutes

Next: Components of a Fitness/Training Program