My understanding is that they talk about swimming as being four times *harder* than running (in terms of calorie burn etc I think), so multiply a swim distance by four to get an equivalent run distance. Therefore a 10km swim is considered ‘marathon’ distance.
This kind of works for me as my first ever open water 10km swim took me almost exactly as long as my first marathon.
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My guts feeling is, 1500m swimming (50min or so), is about 5000m running in the sense of intensity and energy burning. But on the other hand, it took me average 50 min to finish. To run 5k might take me 30min? Or forever.