why wy?

From Medscape (registration needed, free)

The message seems to be that vigorous physical activity is associated with a modestly lower risk of cardiovascular disease when compared with activities of moderate intensity. So if you really really push it, you are not at a higher risk of dropping dead or damaging your heart.

"If two people are expending a thousand calories per week, does it matter if they do that by running or by walking?" said lead investigator Dr Andrea Chomistek (Harvard Medical School, Boston, MA) in explaining the rationale of the study to heartwire . "We found that there might be some benefit to doing it with vigorous activity such as running, but it doesn't appear to be a very strong benefit. As long as you're burning a certain amount of calories per week, between 600 or 1000 calories per week, it's okay if you do that by walking. You don't necessarily have to go out and run a marathon."

So go out there and exercise at your own pace!