Just before Golden Week holdiays began, I swam my first ever 1km non-stop. That is 40 lengths of the 25m pool where I swim. I regularly run 10km for that portion of my workout, and here are some differences I noticed between swimming 1km and running 1km.
It feels longer than it is. When swimming, those lengths of the pool feel pretty long. I am sure for some pro or semi-pro swimmers 1km is no big deal, but for the rest of us it feels very long. It was hard to keep the rhythm throughout. By contrast, while I am sure 1km run feels tough for some, it is not nearly the workout that swimming it is.
Lots of turns. Doing 40 lengths of the pool involved a lot of turning. Very quickly, you cover the distance, and have to turn and kickoff from the poolside. I think this will be better if done in open water, which approximates the way triathletes do it. I'd need to find cleaner water than we have anywhere nearby in Yokohama, though.
Over quickly. I also found the swim to take less time overall than I expected. Running 1km usually takes me around 6minutes (assuming 10km/hour pace). Swimming it took me about 20 minutes, which is actually less than I expected. I am sure distance swimmers go much faster, as I am sure distance runners do as well (making a top mark marathon these days means hitting almost 20km/hour average speeds). For the workout I got, it was over quicker than I thought it would be. This makes it a perfect lunchtime activity, since you could hit the gym, change, swim 1km, and be out in under an hour.
Great calorie burn. I usually burn about 700 calories for a 10km run, which means about 70 calories per km. Swimming 1km seems to burn much much more. I think the best guess is somewhere between 200 - 230 calories. That makes swimming on average about 3 times as efficient a form of exercise as running.
I started swimming frankly because I suck at it. Because I suck at it, I didn't really do it, and thus continued to suck at it. The 1km goal was a good one for me, and was something I could not have achieved 6 months ago. I want to keep swimming as a regular part of my workouts, and next attempt will be a 2km swim. Stay tuned.