Running a marathon is physically hard, but not necessarily for the reasons that some of us might expect, according to a new study of the relative physiological impacts of completing a 26.2-mile race versus a 13.1-mile half-marathon.

The study’s findings provide useful new insights into how runners might best prepare for a marathon. They also underscore possible reasons to consider a half-marathon instead.

By now, runners who have signed up for a fall-season full- or half-marathon race should be well into the training. (I’ve signed up for the Monterey Bay Half Marathon in November.) Our workouts typically involve some mix of long and shorter runs.

But whether our regimen readies us as well as possible for the challenges of distance racing remains surprisingly uncertain, because few past studies have examined what, precisely, makes long-distance running events so demanding, especially for recreational runners.

So for the new study, in the Journal of Sports Medicine and Physical Fitness, researchers in Spain decided to track runners participating in the annual Madrid marathon and half-marathon, both of which occur at the same time and on the same course for the first 13.1 miles.

The researchers hoped to assess what happens to recreational runners physically while running those distances and how any physical strains might affect their performance.

In particular, they were interested in the roles that dehydration and muscle soreness might play in runners’ finishing times.

By contacting registered runners, the scientists recruited 11 experienced runners preparing for the marathon distance and another 11, similar in age, weight and racing experience, enrolled in the half. All were following training programs provided by the race organizers, with the marathoners completing more weekly mileage.

The day before the race, the scientists had the runners visit the lab, where they drew blood and checked for baseline indications of dehydration and biochemical markers of muscle damage.

The scientists also asked each runner to perform several standing vertical leaps. This test indicates how much force the leg muscles can generate. When muscles are tired, people cannot jump as high. The scientists noted the maximum height each runner could achieve.

The next morning, just before the race start, the runners reported to a medical tent where they were weighed and fitted with patches to measure perspiration rates.

Then, they ran. All finished their race, whether it was the half or full marathon. Afterward, they returned to the tent to be reweighed, provide more blood, answer a few questions about how they felt and repeat the vertical leaps.

Finally, the researchers compared data.

The half-marathon runners, not surprisingly, proved to have had an easier time. They had lost body weight from sweating but were not seriously dehydrated. They also had elevated levels of blood markers related to muscle damage, compared to the day before. But their levels were much lower than in the blood of the racers completing the full 26.2 miles. The half-marathon runners also could jump higher after the race than the marathon racers. Their legs, by all indications, were still relatively fresh.

Perhaps most interesting, the half-marathon runners had managed to maintain a steady pace throughout their race. In fact, most had sped up slightly near the end. Their average pace for the final five kilometers tended to be a few seconds faster than for their opening five kilometers.

The marathon runners, on the other hand, had slowed. Most had run at a steady pace for the first 13.1 miles, but then began to brake progressively, so that their final five kilometers were significantly slower than the same distance at the start. Those runners with the highest subsequent blood markers of muscle damage had the greatest decline in pace. They also reported much more soreness in their legs after the race than the half-marathon runners.

These findings, on their face, may seem predictable. Most of us might expect that running a full marathon should be more tiring than striding half as far, and that the more your muscles hurt, the slower you will be.

But, says Juan del Corso, a professor of physiology at Camilo José Cela University in Madrid, who led the study, there were surprises in the data. For one thing, none of the volunteers in either race became clinically dehydrated. So dehydration had not made the marathoning difficult.

More fundamentally, the marathon racers, on paper, should have been prepared for the rigors of the distance, he says. They had run more mileage during training than the half-marathoners. Despite the rigorous runs, however, their legs were not fully prepared for the marathon, he says. Their muscles became slightly damaged and grew sore, particularly in the second half of the race, and their pace slowed.

The implication of these results, he says, is that “just running long distances is not enough to prepare the leg muscles for the great demands of an endurance event like the marathon.” Instead, he says, targeted strength training of the lower body might be needed to inoculate the muscles against the slight tearing and other damage that otherwise occurs during the hours of pounding in a marathon.

Of course, this study was small, short-term and examined only a few aspects of distance running. But its underlying message seems applicable to all of us training for a distance race.

“Gym training,” including the use of machines and free weights to develop strength and power in the muscles of your legs, “can very much help to prepare muscles for the stress imposed by these long races,” Dr. del Corso says. You can also find a fast, in-home, strength-training routine here.