The 'just you and the elements' part is real, in the open water you can't just stop and get out, you have got to complete what you started. The 'completeness' of the exercise is different than biking or walking : every joint and muscle seems to be involved.
Back on land, the exhaustion is a different level than any other. Very deep breaths. When I get home, I am hungry as hell. Chronically under-weight, I seem to be gaining weight, very happy with that.
The end of summer is approaching and the water is getting colder, I am exploring the indoor pools in my vicinity.