Any athlete could probably relate to this. As we get closer to competition, we are working harder and harder; both in quantity of rehearsal time, and in intensity of practice.
Ballroom dancing, as the book title states, is not for sissies. It is physically demanding.
It is not just a matter of moving your own body through three-dimensional space; it is a matter of absorbing, deflecting, or redirecting the forces of your partner's body. The science of that is called mechanics, and that's something any athlete has to understand.
I never thought of myself as athletic growing up. I was always bendy, and for a brief period I was pretty fast. I gain strength quickly, too. But I never participated in a sport (mostly because my hand-eye coordination is crap, and team sports tend to involve projectiles), and my tendency has been to stay "in shape" primarily through diet, not exercise.
The thing that undoubtedly saved me, in terms of being able to take up ballroom dancing in midlife as a serious sport, was yoga. And sadly, the more rehearsal we do the less time I have for yoga. It's one of those downward-spiraling vicious circles.
Mr. P worked on my back for a good half hour over the weekend, getting my right ribs mobile again to take the lock out of my lumbar spine. I paid him back by accidentally bashing his knee in rehearsal. He's got a knot and a half where my patella hit the inside of the proximal end of his tibia. I don't even have a bruise; he is limping. It's not a good place to be, three days before competition.
This is the point at which I (and possibly Mr. P too) always think of ... not doing it again. We are tired. We hurt. Everything else in life has been shoved aside and the undone to-do's are getting snarly.
But like any other athlete, on "race day" the adrenaline will be up. The music will be inspiring, we'll be looking our best; and if we don't absolutely crash, we'll have fun - and then it will all seem worthwhile.