Old CW: Stick to your plan.
New CW: Free your mind, man.
Too many runners treat their training plans as gospel. But a growing number are learning the benefits of "training opportunistically." Here's how it works.
You need to do your most challenging runs on days when you feel good. But you never know how you're going to feel until you start running. Forcing yourself to crank out hard runs on predetermined days means you'll inevitably turn in some subpar performances and won't benefit as much as you could.
Begin each workout day with the option of either a Plan A (challenging) or a Plan B (easy) run. If you're headed out for a Plan A effort but feel flat during your warmup, switch to Plan B: an easy run. And if you find you have plenty in the tank on a B day, gear up to an A. Of course, the catch is the temptation to declare all days Plan B's. Make the switch only if you feel truly lousy. Otherwise, grind through it.
This takes a little discipline. And that's one bit of conventional wisdom that will never change.