Gererally speaking there are considered to be four ‘Heart Rate Training Zones’, Fat-Burning Zone, Weight Control Zone, Aerobic Zone, and Anaerobic Zone, Lets briefly look at one for now, the Aerobic Zone.
What’s the most important muscle in the body, and how is it strengthened?
Your Heart muscle, and is strengthened by performing moderate intensity aerobic activity for 30 minutes, 5 times per week (150 minutes) acording to the WHO (world health organisation), or vigorous intensity aerobic activity for 75 minutes weekly.
But what does moderate and vigorous intensity mean ?
Lets take a 30 year old and a 50 year old as an example.
If the the goal of either age is to significantly improve the strength, functions, and suppleness of the heart, lungs, arteries and associated tissues then raising your ‘Heart Rate’ during moderate aerobic activity/exercise to 135 beats per minute approx, during your activity/exercise for your recommended 150 minutes per week will achieve this. Alternatively raising your heart rate to 155 beats per minute approx,  for 75 minutes of vigour intensity aerobic activity weekly will also achieve similar benefits.
As we age our maximum heart rate lowers, so a 50 year old’s target heart rate training zone should be 120 beats per minute approx for 150 minutes of moderate aerobic activity/exercise weekly, or a heart rate zone of 140 beats per minute approx for 75 minutes of vigorous aerobic activity/exercise.weekly.

NOTE: The WHO also recommend that resistance exercises should be performed twice per week.

THE GOOD NEWS is that a good Circuit Training class twice weekly, added to a couple of power walks/jogs/runs weekly will achieve this.
Attached are some SSF June and July Circuit Training photos.