His doctor recommended a regular exercise routine, so Mr. Sherwood, then 58, began running. He continues to run on the treadmill a few days a week and even regularly works out with weights.

But Mr. Sherwood began to have second thoughts about this exercise regimen last month when he learned about the death of "Jogging" Joe Shafran, who had written a column about running in The Capital. Mr. Shafran, 81, was running on Jennifer Road when he collapsed and later died.

Mr. Shafran's death made Mr. Sherwood wonder if at his age - he's 72 now - he could be too old for such a workout.

"As time goes on, I'm pretty sensitive to people having problems while exercising," said Mr. Sherwood, of Annapolis. "My heart attack was very serious. It didn't knock me down, it didn't knock me out, but it could've in another hour if I hadn't gotten to the hospital. Now I'm at a point where I am a little worried about stopping exercise because quite frankly it's pretty hard and it's getting harder."

But doctors said just like any workout routine, running is good for the body, but must be stopped if it starts to hurt. To prevent injuries, runners 40 and older need to slowly increase the intensity of the workouts, stay hydrated and adjust expectations.

The sport is not just for the younger set. RunningUSA.org predicts that the population of runners 45 and older will increase more than 40 percent by 2020. Female runners 60 and older made up 3 percent of timed-road-race finishers in 2006, the latest statistics available on the Web site. Men in the same age group made up 6 percent of runners in that same category.

No matter what the age, anyone thinking of starting a running program should check with their physician, doctors suggest. But seniors can easily see the benefits of it as quickly as younger runners, said Dr. Jon Altschuler of Cardiology Associates.

"I know a lot of folks over the age of 65 who run and do it very regularly," said Dr. Altschuler, an avid runner who also practices at Anne Arundel Medical Center. "It's terrific exercise. Some of the most vital, healthy seniors that I get to see in my practice are people who exercise very regularly. I think it has much to do more to do with one's quality of life as it does for anything else that is out there."

Don Singer took up running as a hobby 16 years ago, when he turned 57. It started out as something he would do occasionally to clear his mind, then he began entering in races and marathons. Since he started running, Mr. Singer, now 73, has competed in five marathons.

He said the constant exercise also has helped him keep his weight down.

He said he has received differing opinions from doctors over the years. One cardiologist supported Mr. Singer in his runs, but another one said that Mr. Singer should not be running competitively.

He stopped seeing that cardiologist.

"When I go out on a practice run, I'm always competing against myself," he said. "If I did a certain time last time, (I try to beat it). (Running) keeps my weight down. I think it's also good for me mentally as far as repressing anxieties and things like that."

Dr. Altschuler agreed. He once saw a 72-year-old man complete a triathlon and said he hopes to do that when he reaches that age.

"Cross training is a wonderful thing," he said. "For me, there's a runner's high that makes you feel good … I feel much more vital when I'm exercising regularly."