Dr Biddle said 30 minutes of brisk walking each day would bring substantial health benefits.

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The government recommends the equivalent of 150 minutes of brisk walking or other moderate exercise per week.

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These recommendations include at least 60 minutes of moderate-intensity aerobic exercise a day, such as brisk walking, plus high intensity activity three times a week.

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Research by Arthur Kramer, a professor in the department of psychology at the University of Illinois at Urbana-Champaign, has shown that when sedentary aging adults, ages 58 to 78, take up a regimen of brisk walking they can improve their ability to ignore distractions.

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The 2008 Physical Activity Guidelines for Americans released by the Department of Health and Human Services recommends that adults age 18 to 64 engage in at least 2.5 hours of moderate-intensity aerobic exercise (brisk walking, biking) or an hour and 15 minutes of vigorous activity (like running) each week.

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Determined to run at least a little of the course, I started as I knew I wouldn't be able to go on for long, jogging until the crowd thinned out and I could take up the brisk walking pace I hoped would get me back to the stadium before the street cleaners moved in to sweep the course.

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