That's the advice of a BBC article:
http://www.bbc.co.uk/news/health-17028712
Unfortunately the article does not define what constitutes fast or slow walking.
It does link to an earlier study which is available online and does define its
protocol. To divide into approximate thirds it uses speed ranges of <1.5 m/s,
1.51 - 1.84 m/s, and >=1.85 m/s (these are for men 65+. with slightly lower
limits for women)
http://www.bmj.com/content/339/bmj.b4460.full
The middle and fastest group perform about the same, but the slowest group has
increased risk of heart attack and stroke.
They measure the speed over a short course. The first 3 m are unmeasured to
avoid measuring acceleration, subject is timed from 3 m to 9 m (6 m total)
This makes it hard to relate to exercise walking. I can maintain 1.72 m/s over
3.6 km, but based on the people I pass, I feel I'm in the fastest third not the
middle.
