[chrismar]

Aerobic fitness does not help static breath hold at all. In fact it may be actively detrimental.*

The reason for apnea walking, etc is that it is close to pure anerobic lactic training. Very little other training comes close. Running, cycling etc are all primarily aerobic, with some anerobic alactic and a bit of anerobic lactic thrown into the mix. There is a complicated physiological answer to the question, but the simple answer is that the kind of fitness you'd build up in other sports isn't nearly as helpful in freediving as you might think.

*This is my opinion, from my limited personal experiences and observations.