I doubt the "general health" benefits of 45 minutes of exercise a week are significant.
it's Dr. Kenneth Copper's recommendation actually
According to Dr. Cooper, there are two levels of training that we are looking at here:
1) Health and Longevity Training
2) Aerobic Training
Health and Longevity Training is defined as 30 minutes, collectively, of activity, regardless of the heart rate response. This means moving, 20 minutes 4 times a week or 30 minutes 3 times a week to provide health protective and wellness measures for – you got it, increased health and longevity.
In contrast Aerobic and Cardio Training means you must keep the heart rate elevated to between 65% and 80% of the maximum heart* for those same periods of time: 20 minutes 4 times a week, or 30 minutes 30 times a week.
The difference here, according to Dr. Cooper, is the deeper intensity with the Aerobic Training – also called Cardio – has a more dramatic impact on slowing the age response, accelerating weight loss, and is a rule of thumb preventative extra measure for disease prevention.
http://lanimuelrath.com/confused-about-cardio-my-conversation-with-dr-kenneth-cooper/