If you're tired, take a day off. That said, what matters isn't mileage so much as time and intensity. If the 45 miles on Sunday is sitting in on a coffee shop ride, that's a lot different than 45 miles of racing. That's why Friel developed the training stress score.

Personally, I prefer an easy hour of light spinning to taking a day off. You still get outside, you have active recovery, and you still get yourself to work.