I started doing intervals in youth track almost forty years ago. They hurt. When I was preparing for a triathlon almost twenty years ago, I read that swim intervals would make you faster. After reading "The Cyclist's Training Bible" by Joe Friel, I had to admit that intervals would make me a faster racer and I have been doing intervals in preparation for racing for three years now. They still hurt. I know they work, but that doesn't make them hurt less. I am at that point in my training plan (yes, I have a plan) where I have to get serious about intervals. They hurt. That's all I'm going to say about that.