Being flexible with your plan has several benefits. It gets you to tune in...be aware...listen to your body. You get into a dialogue with your physical self...rather than imposing the stubborn will of the mind.
Being willing to quit a workout doesn't mean that you will. It just means that it is an option. It shows wisdom.
Paradoxically...it has been found that having the option to quit actually results in us doing my work. This article does a great job of describing it and cites a study which concluded that having the option to quit resulted in more work.
Be tenacious...but still keep it loose and light.