Your Second Wind

I also think I killed that voice, but it was quickly replaced with another voice.  This one starts chiming in after the workouts and usually says things like "you are way too old to be doing this to your body.  Did you forget you aren't 18 anymore?"

I think this is for people who need extra motivation.  some of us will beat ourselves up with intense exercise until we're sick or injured (or both)... repeatedly.

my problem is that i think my body is tougher than it really is. 

I don't think of second wind or third wind but as "valleys" that one goes through (times when your energy is low in a long workout or race). This concept is courtesy of a former coach. You get into a valley and you think there's no way out, but then you come out and there's another plateau, then another valley. And if you keep going through the valleys (maybe taking in more drinks or nutrition, fixing your form or breathing, talking yourself out of quitting), you will find you can go a lot further than you think. He was an ex-marine and told us to think about the Marines' prayer--"Yeah, though I walk through the Valley of Death, I fear no evil for I am the ***** in the valley." It usually works for me (probably because it makes me laugh, and forget how much I'm suffering), YMMV!