for any person to travel 10KM or 16MI in a dive useing only your legs and also

what do you do when your out of breath in a dive?

I think what the asker meant by out of breath is being winded on a dive, not actually out of air.

What you do is stop, relax, and control your breathing. Get your breathing back under control, then continue your dive. Monitor your air closely, as being winded probably put a good dent in you supply.