ANother factor. I am a scuba diver. After I stopped running, my muscle tone in my legs went to heck..I recall diving off a point of one of California's channel islands..A really strong current came along..Without a "Sea Palm" to cling onto I would have been involuntarily heading south.. After I got out of the current, I swam like hell. Soon afterwards the legs staring cramping like crazy.. Being out of shape and a scuba diver is not a good idea.
Since biking I feel more comfortable in the water.