It's one of the hidden clauses of the two hours per day rule.
If you do three hours a day on day one, that does not mean you do only one hour on day two. The requirement is always two hours a day. Anything more is bonus, but does not take out of what you have to write the next day.
If you do six or seven hours on day seven, that does not mean you do six or seven on day eight. And don't you dare feel bad about yourself if you can't do more than one or two long sessions. Your goal is two hours a day.
I believe this method reduces burn out; you write longer because you want to, not because you have to. Your muse knows where to find you and when, and you know it has really hit when those sessions make their appearance. It's exciting.
One caveat: You may still find yourself burning out down the road, and you may be tempted to blame your longer sessions for it. But I don't think that will be the true cause, as long as those long sessions happen naturally.