That fine line we all walk, when to cruise and when to battle through and push.
Training smart is all about developing fatigue (and large amounts of it sometimes) and then letting your body recover and adapt to the stimulus you applied. Repeating this cycle allows you to become faster, stronger, and handle more volume. The trick is knowing how to periodize your training and give yourself proper recovery when you need it. If you don't allow for enough recovery your body will force you to take it, this can be in the form of sickness and/or injury. Timing these breaking points is key for big races and tapering. If you time it correct you can get a perfect supercompensation and crush life.