Despite its dangers, I love walking. From the WaPo:

If a pill could significantly lower the risk of heart attack, diabetes, stroke, osteoporosis and breast and colon cancer while reducing weight, cholesterol levels, constipation, depression and impotence and also increase muscle mass, flatten the belly and reshape the thighs even as it reduced the risk of age-related dementia and made you better-looking -- and had no negative side effects -- there would be panic in the streets. The American economy would tip into chaos. The military would have to be called in to secure supplies of the medication.
Luckily, there is no such pill.

But a large and growing body of credible research demonstrates that taking a good walk most days of the week can deliver all of the health benefits cited above and more (although we admit the "better-looking" part is harder to prove).

Of course, the sniper attacks put a crimp in walking. Still, I bet more Washington area residents are in danger from being overweight and under-exercised than from a sniper's bullet.

You can tell these guys don't really like walking.

All this activity causes the brain to release endorphins into the bloodstream. Endorphins, which have chemical properties similar to opium, are responsible for blocking pain and ushering in that cozy sense of well-being you feel as soon as your walk ends.

My endorphins start to kick in almost as soon as I start. Help! I'm in danger of addiction to something like opium!

Update

A study in the Journal of the American Medical Association suggests men who engage in high intensity exercise (running or jogging at 6 mph) combined with weight training for 30 minutes or more weekly reduce their chances for heart disease. Low-intensity activities (walking at a pace of about 2 mph) is less effective.