I've built up a solid base of running over the past few years, up to consistent 80/90 mile weeks. However, I want to start incorporating more speedwork/mile-specific training while I am still relatively young (22 years old). I've looked at Pfitz/Daniels, and their lower-distance training plans max out at much lower mileage (50-60 mpw). I know that incorporating more intense runs may mean that I need to sacrifice some mileage, but I do not want to lose the base that I have built up. That being said, has anyone tinkered with these plans by adding in more mileage, whether it be to the warmup/cooldown of intense days, easy runs, or even doubles?