John,
Some pointers:
- I would wait 24h after a hard cardio session before trying to do a maximum breath-hold swim
- I would assume that for your exam you are not allowed to use any sort of weightbelt, neck weight or suit? If so, then you must train in exactly the same configuration that you will be tested with.
- You can learn to pack your lungs, gettings as much as 3 litres of extra air; however this will make you VERY buoyant, which may cause problems since I assume you can't use a weight belt
- To adjust your buoyancy, recall that the depth you swim at affects your buoyancy-- the deeper along the pool that you swim, the more negative you get, so if you are finding yourself buoyant, swim along the bottom as closely as possible
- When doing the maximum swim, it essential that you are quite cool; almost cold, but not shivering
- It essential that prior to starting the swim, you must not move for at least 5 minutes. It is hard or impossible for your body to store oxygen and energy if you are moving about in any way during the 5 minutes prior to the swim
- In that sense, I would say the best preparation would be to lay down motionless, near the edge, for at least 5 minutes, if you are allowed to do so
- By practicing with a buddy, and following the above tips, you should blow past the 50m goal. Try to impress your instructors by doing 100m! Keep in mind that the world record (without a wetsuit) is 166m, although that was done with a neck weight.
- The more deeply you breathe before the swim, the better your lungs will feel, but this will reduce your ultimate distance. The urge to breathe, however unpleasant, increases the distance at which you will ultimately black out. If you are finding that the urge to breathe is what is stopping you, then breathing a bit more before hand may help (always with a buddy!). On the other hand, if you are finding that having blackouts or sambas (shaking) is your limiting factor, then breathing less before hand will help.
- Your stroke mechanics will have a huge effect on distance. Try to glide as much as possible, using fewer strokes to cover the same distance. Try to relax and go limp during the glide phase. Every muscle that is clenched is burning precious O2.