Courtesy of WOWSA, Huntington Beach, California.

After observing some of the world’s fastest swimmers perform around the world in oceans and lakes, we wondered what is the equivalent of a 25 km (15.5-mile) marathon swim on land?

How much do you have to run, jog or walk before it is the dryland equivalent of 25 km?

That is a tough question to answer. While it is never a perfect comparison and is dependent upon each individual’s running ability and their degree of comfort in the open water, one body of opinion is that 1 mile of swimming is equal to 3.49 miles of running. That comparison may not be accurate for everyone, but a relatively fast swimmer can swim a mile in the ocean in 20 minutes and a relatively fast marathon runner (who can run a 2 hour 30 minute marathon) can covers 3.49 miles per 20 minutes at a 2 hour 30 minute marathon pace.

So if that is one comparative case of an elite open water swimmer and an elite runner, then a 25 km swim could be said to equal to a 87.25 km run. In other words, a 25 km open water swim could be said to be easier than a 100 km run – and we would bet that the recovery time would be much faster for the elite swimmer versus the fast runner.

Of course, if the water temperature is low or the conditions are rough, then the swimmer’s distance is reduced and the equivalent running distance will increase. Conversely, if the air temperature is high and the runner is facing an oncoming wind, then the runner’s distance is reduced. For example, if the water temperature went from 20°C (68°F) to 10°C (50°F), then swim would become much more difficult or if the air temperature went from 20°C (68°F) to 30°C (86°F), the run would become more difficult.

But how is difficulty in endurance sports measured? What is difficult for one endurance athlete in one type of venue and set of conditions may not be the same for another. Frankly, it is impossible to measure and compare difficulty across the broad spectrum of humanity.

Conversely, how many people in the entire world can swim 25 km – at whatever pace? Compare that number to the number of people who can run 100 km – at whatever pace.

If we assume only 20,000 people in history have swum over 25 km at one time, the number of people who can swim 25 km is a relatively small number. The number of people who have or are capable of running (or walking) 100 km is much larger.

So what is the comparable land distance of 25 km in the open water?

Opinions are greatly appreciated here.

Upper photo by Elias Lefas shows Greek open water swimmer Antonis Fokaidis. Lower photo by Dr. Jim Miller shows the 2010 World 25 km Open Water Swimming Championships.

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