There are voices in our mind, talking to us all the time. They're trying to dissuade us from doing the hard things, always encouraging us to take the easiest path. You hear them when you're exercising, and they're telling you: you're in pain. Stop now. You already did a good job.
We all have those voices, but we can learn to subdue them, and we can choose not to react to them. It's a habit that you can ingrain with constant practice. Some days when you wake up you feel full of energy and motivated to exercise right away. Other times you wake up not feeling as good and even before you start, the voices start finding excuses to not run.
These thoughts that your mind keeps presenting are just waves. Like waves, they come and go, they're there for a few seconds then they might disappear. As you start exercising, these waves will become smaller and smaller until they vanish.
When these waves are trying to dissuade you, it's as if they were planting weeds in your mind, trying to absorb your motivation. However, it's important that you don't avoid or get angry at them. In Zen meditation, it's said that "Pulling out the weeds (voices), we give nourishment to the plant (exercise)". So even if you're having some difficulties facing these weeds, be grateful that they are there because this is how we improve: by confronting and winning over our doubts.
Once you have some experience with transforming the weeds in your mind into mental nourishment, you will start to make great progress with your goals. You'll start recognizing these voices, allowing you to confront them and use them to enrich your mind. This is the key to learning self-control and discipline.
Of course, the actual learning comes from experiencing this first hand. So welcome these voices next time they come at you, try to observe them, and then bypass them and start your exercising session.
When you're exercising and a voice tries to persuade you to finish early, telling you that you've done enough, or are way too tired, do this. Hear it, observe it, and then remember this principle: Strive to the end. Always complete your routine: if you're tired, run in place. If you can't run, walk. If you're too exhausted, rest, and then resume your workout, but stick with it until the end.
If you make sure to finish what you start time and time again, you will start acquiring a new mindset: I need to stick with it, even if I have to modify my routine or rest a little bit, but I'll keep going to the end. Quitting is habit, but so is persevering.
Try to exercise, even with these troubled voices. Breathe, run, and empty your mind. Next time you feel like quitting, remember to pull out that weed. Always finish your exercise session. The way you address yourself will change over time. Soon those loud voices will become smaller and smaller until they become a little voice you can't even hear.
Before and during your exercise try to listen to your mind weeds. How are they trying to discourage you? Can you observe them and choose not to react? Don't abhor those voices, learn to appreciate them and use them to nourish your self-discipline. Through hardship, we build character.
(Excerpts from Letter No. 4 Mind Weeds from the android app Fabulous - Motivate Me!)