OW vs. Marathon

My wife being a runner asked yesterday after I completed a 2.3ish mile swim what is the equivalent swimming-wise to a marathon. Because she had always assumed that a 2.4 mile swim would be like running a marathon since it's part of the Ironman Tri. I didn't really feel like completing that swim that I would compare it to a marathon. While I was pooped and sore, it was not like having my muscles ripped to shreds and hurting to move several days. Any thoughts? Also the other discussion we had was the fact that in a running or biking race, you really are running/biking that distance, but in a swim you are doing some n amount more than the straight distance. For swimming your fighting the waves and your own ability to swim straight that you do some amount more. I've thought "how great would it be if I could place a gps tracker around my ankle that I could plot my swim and see how much I really did."