Not Medical Advice: The significance of starting from low level fitness is to give the body time to adjust before going to moderate where stamina is build.
There are a couple different ways to measure the level of intensity at which you are exercising and that level is based on your individual fitness level and overall health.
The Borg Rating of Perceived Exertion (RPE)
As defined by the Center for Disease Control and Prevention, this is a way of measuring physical activity intensity level. Perceived exertion is how hard you feel like your body is working. The RPE is based on the physical sensations you experience during physical activity, including:
increased heart rate,
increased respiration or breathing rate,
increased sweating, and
muscle fatigue.
A high correlation exists between a person's perceived exertion rating times 10 and the actual heart rate during physical activity; so a person's exertion rating may provide a fairly good estimate of the actual heart rate during activity (Borg, 1998). For example, if a person's rating of perceived exertion (RPE) is 12, then 12 x 10 = 120; so the heart rate should be approximately 120 beats per minute.
Note that this calculation is only an approximation of heart rate, and the actual heart rate can vary quite a bit depending on age and physical condition. The Borg Rating of Perceived Exertion is also the preferred method to assess intensity among those individuals who take medications that affect heart rate or pulse.
Self-Monitoring
During your workout, use the RPE Scale to assign numbers to how you feel. Self-monitoring how hard your body is working can help you adjust the intensity of the activity by speeding up or slowing down your movements.
Through experience of monitoring how your body feels, it will become easier to know when to adjust your intensity.
Moderate-intensity physical activity is defined as - physical activity done on a scale relative to an individual's personal capacity, moderate-intensity physical activity is usually 11-14 on a scale of 1 to 20.
Vigorous-intensity physical activity is defined as - physical activity done on a scale relative to an individual's personal capacity, vigorous-intensity physical activity is usually 17-19 on a scale of 1 to 20.
Get more info on knowing your level of intensity at Heart.org and BellaOnline.com.