when do you catch your second wind?

I know going into every workout that the first 1800 yards are going to be torture for me. My lungs feel like they're going to explode, my joints feel stiff and concrete and the water seems as thick as honey. But I also know that once I reach 1800-2000 yards I'll find my ryhthm and everything will fall into place. Where is that at which you feel at one in the water?