A short book with one idea pursued patiently: most worthwhile pursuits have a long, uncomfortable middle, and most people quit during that part.
The harder skill is knowing the difference between a dip worth crossing and a dead end pretending to be one.
I liked the running analogy. Quitting because running feels uncomfortable is usually not a good reason. Quitting because your form is breaking down, the injury risk is rising, or the long-term cost no longer makes sense is different.
The book is less about persistence at all costs and more about strategic retreat. Knowing when discomfort is part of the path, and when it is useful information.