People are seriously underestimating the difficulty of swimming on this board. I was running 5k+ a day in around 20mins, and made the switch to swimming. I struggled hard for the first week or two to manage even half a mile, it's a different level of exercise altogether, far more beneficial than running.
I'd say a mile, if you push hard and work different strokes, is equivalent of at least 5k in running. At least. If you really work it, it can be even more, as it works every part of your body, without the kind of pressure running puts on your joints.