Startlingly, a new British study, as reported by the BBC, suggests that just five minutes of outdoor exercise provides a measurable health benefit:
UK researchers looked at evidence from 1,250 people in 10 studies and found fast improvements in mood and self-esteem. . . .
The research looked at many different outdoor activities including walking, gardening, cycling, fishing, boating, horse-riding and farming in locations such as a park, garden or nature trail.
The biggest effect was seen within just five minutes.
With longer periods of time exercising in a green environment, the positive effects were clearly apparent but were of a smaller magnitude, the study found.
Looking at men and women of different ages, the researchers found the health changes – physical and mental – were particularly strong in the young and the mentally-ill.
And Physorg.com, a science website, reports that cardiovascular fitness can be improved, and the risk of diabetes reduced, by brief, high intensity workouts:
A regular high-intensity, three-minute workout has a significant effect on the body’s ability to process sugars. Research published in the open access journal BMC Endocrine Disorders shows that a brief but intense exercise session every couple of days may be the best way to cut the risk of diabetes.
According to the article:
The subjects in this trial used exercise bikes to perform a quick sprint at their highest possible intensity.
Hmm. Five minutes of outdoor walking combined with three minutes of sprinting.
No more excuses not to exercise?
Prometheus Unbound 
If we can’t sprint or slow jog. Try brisk walking. It will improve your cardio.