Recommendations
So how much exercise should a person do.... It really depends on the type of activity (cardiovascular exercise, muscular strength, muscular endurance, or flexibility) as well as what the person's goals are (health, weight loss, and many others).
As a general recommendation, the American College of Sports Medicine (www.acsm.org) and the American Heart Association (www.heart.org) worked together to create an amount of activity a person should do in order to become healthier (meaning how a person can lower his/her chance of getting a chronic disease).
The recommendations can be found on this page:
A little more detail can be found here:
Moderate vs Vigorous Activity...
In the recommendations, the ACSM and AHA talk about Moderate- and Vigorous-Intensity of exercise. The differences between these two levels of activity is described on this webpage (Click Here), scroll down to the section that is labeled "Gauging intensity by how you feel".
... and Examples:
You can also get a feel for the difference between these two levels of intensity through a Center for Disease Control (CDC) webpage that provides examples of various activities (Click Here for CDC webpage)
Current State of Americans in following the Recommendations:
In the Physical/Mental Benefits of Exercise part of Section 1, you went through some data from the Health People website. The government also provides data on how many Americans are exercising enough to meet or exceed these recommended values. One of the 2020 goals is that 47.5% of Americans will achieve the Moderate or Vigorous activity level recommendations has already been satisfied. But there are still more people who need to try and achieve these minimal values.
Notice the use of "minimal values" in the last paragraph. The activity guidelines are minimal values, in other words... what can a person do minimally to try and get healthier. If a person does more than these values, he/she will see even more improvement when trying to reduce his/her chances of getting a chronic disease.