Unexpectedly, the death rate remained high among the slowest walkers, even if they met or exceeded the standard exercise guidelines and expended as much energy per day as someone walking briskly for 30 minutes. This effect was most pronounced among the slowest of the slow walkers, whose pace was 24 minutes per mile or higher. They were 44 percent more likely to have died than walkers who moved faster, even if they met the exercise guidelines.

One important inference of these statistics is that intensity matters, if you are walking for health. "Our results do suggest that there is a significant health benefit to pursuing a faster pace," Dr. Williams said. Pushing your body, he said, appears to cause favorable physiological changes that milder exercise doesn't replicate.

So check yours, your spouse's or perhaps your parents' pace. The process is easy. Simply find a 400-meter track and, using a stopwatch, have everyone walk at his or her normal speed. If a circuit of the track takes someone 6 minutes or more, that person's pace is 24 minutes per mile or slower, and he or she might consider consulting a doctor about possible health issues, Dr. Williams said.