Over the last 1-2 years, I've been consistently running 75-90 mile weeks, but rarely on a structured program and going only by feel, similar pace daily, which I know is not ideal for training. I want to start introducing more quality sessions and follow a plan such as Faster Road Racing. However, I really like the base I've built and do not want to severely decrease my mileage, although I am aware that the introduction of more consistent speedwork will lead to more fatigue. Is there a certain percentage that would be recommended for me to take off of my mileage, or should just the slow introduction of 1 speed/quality session a week without sacrificing mileage be enough to boost my training?