Intervals and other types of speed work are essential to improve running speed.
You have to know your body. It's part of the beauty of the training process, and once you've determined how much your body and mind can take, you can then begin to reach your potential.
To put it another way, Michael Jordan was a gym rat.
I also held several masters running titles.
There's always the feeling of getting stronger. I think that's what keeps me going.
I want my time spent running to serve as a reward.