I recently started swimming since I'm planning on doing a triathlon next summer and it is by far the hardest of the three disciplines. I started out not being able to do more than 50m without having to catch my breath and this was after an entire summer of running and a month of road biking, so it's not like I wasn't in decent shape. Swimming is a whole different ballgame. I'm now at the point where I can swim indefinitely (did 1500m with two 20 second breaks at the 10th and 20th laps recently) but it took a while to get there. Like anything, it's all about your form. I had to learn how to settle down and swim slower, gliding through the water, controlling my breathing, and reaching on every stroke. Once you get the form down you can pretty much go and go, but it's hard to trust that you won't start sucking water in when you get too tired. I find it helps to really exaggerate your rolling motion so that your mouth clears the water more easily when you go to take a breath. It also helps you reach your arm out a couple extra inches.

The most surprising aspect of swimming was how ripped my chest and arms felt afterwards. I've been an avid weightlifter since high school and it's the same feeling I get after doing bench presses or fly's.