We all have cycles. Some days we're up; some we're down. It's just the nature of things.
However, I also believe that we can influence the ups and downs. We want to be sure to create patterns that are going to generate ups and not patterns that will generate downs.
What I had to say about pacing was about looking for that point at which the pattern is generating an up. By treating myself easily, while still pushing, I created an up. Push too hard, get injured, and I'm generating a down.
We can focus on the downs or on the ups. This focus tends to create more of the same. If we focus on what's wrong, it's easy to feel worse. If we focus on what is good, then it's easy to feel better. And that's not got to be a matter of turning a blind eye--but rather a question of how long the seeing eye remains focused, and on what. I really enjoyed the movie "What the bleep do we know;" I think it expresses these ideas well.
There will be ups and downs. But can we shift the baseline? I believe so.