Manu: the key to long term treading is to relax. Don't kick hard, don't flail your arms to keep your head above water. Use slow, rhythmic kicks and keep your arm strokes smooth and in time with your kicks. What I mean by this is from the full back position, your arms should be straight but not locked as they go forward, and on the back stroke, your elbows should be bent 15-25 degrees, witht he palms of your hands pointing in the direction of the stroke. For the kicks, your legs should be mostly straight, your toes pointed. This should keep your chin at or just a millimeter below water level, but your nose and mouth will be above water level without worry. Also, KEEP BREATHING!!!!! Slow, long, deep (to-the-belly) breaths will keep your natural buoyancy up, as well as keep your body oxygenated. Do that every day until you can go 3 minutes, and you'll be fine

Also, remember, a lot of it is mental. BELIEVE that you can do it, and within a few short days I expect that you'll see your time drastically improve
