Whenever I first get in the water and start going at swimming, I always go through three stages in the water.

The first lasts just minutes, and it is just my muscles not being prepared to work, and still getting warmed up, thus the aptly named "warm up". This one is a little tough, but I know it won't last long.

Stage the second is the glucose stage. This is where your muscles start using sugars they have stored up to create more energy for you to move. This stage is more stable than the first stage, as you feel stronger, and move better than the first stage.
However, it starts getting harder and harder to maintain pace. The glucose is running out.
This is when you have to use your "switches", thoughts that motivate you.
For me, I like simple. Hot boys usually works pretty well (Nothing like a hot boy to get that adrenaline going :D) or just thinking of Sectionals, or something like that.
This is the hardest part, and sometimes I get really nauseous, and get side aches, and cramps. Ugh.

Finally, you reach what I call the Equilibrium.
This is where you start utilizing the fats you have stored up.
It feels GREAT.
For example, my swim coach told me today that on one of the sets we did, 36 50's, she told me we did the first twelve 42-46 seconds, the second, 41-43 seconds, and the last twelve 36-40 seconds.
So, even though the last twelve were by far the fastest, they were also the EASIEST. I had reached the Equilibrium.

Once I have reached the Equilibrium, I feel like Superman. I feel like I could run 10 miles. It is, however, very hard to reach, as it takes generally over an hour swimming, and probably 1 1/2 - 2 hours running.
But it is worth it.