When I first started riding I found I was constantly pushing to improve. Heart racing, sweating, on and on. Mainly because I hadn't noticed that I HAD been improving. One day I decided to take it easy, and have just a leisurely ride; to hell with my time, I was getting tired of my burning legs. While that morning was slower than the day prior, I was still faster than when I started, and I barely even exerted myself!

I know that's not technically the answer to your question, but everyone else had that covered. So the moral of my story is...once in a while take the time to take it easy and just enjoy the ride and the breeze in your face, and appreciate the "advances" you've made all at the same time. Go out charging at full steam another day.