An analogous comparison: someone that has to wear a knee brace to exercise would not be said to have strong knees.
Sufficient sleep plus exercise could substitute for all of them, but then I wouldn't have more than two hours left for work.
I didn't say they couldn't. But they don't always, for one reason or another. We're flawed, we have bad habits, we have vices, we have disordered tastes, etc. These can steer us away from the good toward destructive ends, even though they may provide us with a cheap and empty source of fleeting pleasure.
> If there is insufficient pleasure from work, it's typically because it's insufficiently good.
Continuing the thought above, there are plenty of things we know that are good for us that we nonetheless don't want to do. We struggle to do them, but this does not mean they are insufficiently good, not in the least! It means we are not sufficiently good. Perseverance (or stamina, to use the author's terminology) means enduring unpleasantness, even suffering, for the sake of the good. The more perfect a human being is, however, the more pleasure is aligned with the good, because a more perfect human being is better aligned with the good. Indeed, in a perfect human being, if the situation demanded it, suffering and dying for a worthy good would be a pleasure.
What is experienced as pleasure is not fixed and determined only by the object, but also conditioned by the subject. The reason is similar to the difference between good and bad taste. Good taste is an alignment with and receptivity to the objective good, while bad taste is rooted in dullness, or a disorder of receptivity, or whatever.