An estimated 25,000 runners will embark tomorrow on a 26.2-mile run from Ala Moana Beach Park to a finish line in Waikiki for the Honolulu Marathon.

But when it comes to the cardiovascular benefits of running, are marathoners who go the distance really better off than, say, someone who maxes out at 10 miles, or even five miles at a time?

There's no question that regular exercise can improve your health, but running 26.2 miles may be more about guts and glory than health.

One of the best ways to avoid health-related problems or injury during a marathon is for runners to understand the risks they may face and the precautions they can take.

CHALLENGE, RISK

The marathon is traced back to ancient Greece, when the legend of the sport, Pheidippides, ran from Marathon to Athens to inform the Athenians about the outcome of a battle. Upon completion of his 26-mile trek, Pheidippides is claimed to have staggered in exhaustion and exclaimed, "Rejoice! We conquer," then collapsed and died on the spot. Out of this event was born the most audacious of races.

As centuries passed, the marathon took on a life of its own, and people from around the world have been daring to conquer it ever since.

Nearly 500,000 Americans run in marathons each year. Most people who run marathons are, in fact, not professional athletes but ordinary people running for charity, the memory of a loved one, for a spouse who is sick, or simply for the thrill of being able to fulfill a lifelong dream.

As Pheidippides so dramatically illustrates, however, running this great distance can place a strain on the body.

Among marathon runners, the greatest cardiovascular risk seems to be among people who are first-timers, those who train the least and those who have undetected cardiovascular conditions. Newcomers who do not train properly for a marathon are embarking on a journey of sudden, strenuous exercise that may put them in danger of a potential heart attack.

Dr. Arthur Siegel, director of internal medicine at Harvard University's McLean Hospital in Belmont, Mass., and a seasoned marathoner, says that "running a marathon leads to skeletal muscle injury, which puts the body on alert, and can make the blood more ready to clot."

This, in turn, can trigger a heart attack, Siegel warns.

Experienced runners understand the ebbs and flows of their bodies, as well as the nutritional and fluid requirements needed to obtain optimal performance. But for ordinary people, finding that balance point can be hard. And pushing the body to an extreme can be foolhardy if a runner is not educated and prepared.

KNOW THE DANGERS

Dr. Paul Thompson, director of cardiology at Hartford Hospital and professor of medicine at the University of Connecticut, outlines other potential complications from taking on a marathon: "Overtraining for a marathon can bring about such symptoms as chronic fatigue, trouble sleeping, muscle fatigue, nagging congestion or sore throat, persistent aches and pains, and even depression."

Factors affecting your ability to make it to the finish line include heat, humidity, exposure to direct sun, wind and altitude. Thompson says these environmental factors can place more stress on the cardiovascular system. "Your heart has to pump blood to the muscles to run as well as the skin to cool off, which increases cardiovascular stress," he says.

Cardiovascular risks can even occur after a runner has finished the race — "this risk is increased for at least an hour after exercise," Thompson says.

Even drinking too much water before and during a marathon has become a health risk for runners. The ideal recommendations for fluid intake are the subject of much debate. Too little and you risk dehydration; too much and you risk overhydration, also known as exercise-associated hyponatremia.

The American Medical Athletic Association points out some of the warning signs of overhydration: puffiness (swollen fingers, tight-fitting watch), nausea, vomiting and a progressively worsening headache, all of which can lead to more serious complications. Siegel warns that over-hydration can result in brain injury from cerebral edema, which can lead to the cardiovascular system going into overload, causing pulmonary edema.

RECOMMENDATIONS

On race day, it is recommended that each marathon runner print name, emergency contact information, address, phone number and details of any medical conditions, allergies and current medications on the back of the runner number.

How do you know if running a marathon is right for you? "Genetics play a big role in determining athletic potential," Siegel says. "We can't all be Olympians."

Knowing you've educated yourself about preparation, hydration and expectations also is key.

Training to reach a state of maximum ability and optimal endurance is worth the effort. The reward can prove to be a lifealtering experience that will improve you physically and emotionally. However, "persons with risk factors should be spectators at marathon races if their goal is a long and healthy life," Siegel says.

For Paula Vergara, of Watertown, Mass., the Boston Marathon is her hometown race.