I'd also check online for estimates of calories burned. Probably best to judge by time and perceived exertion than distance. Your stroke efficiency will affect how much you would burn per any measure of distance. Someone who has poor stroke efficiency will burn far more calories over that same distance than someone with better stroke efficincy. However, at the same weight/height/body composition they might burn similar amounts of calories in the same amount of time at the same level of exertion, but the person with better stroke technique will swim a greater distance in that time.
Hope that made some sense.
