A: Some people love to push hard. I am not one of them so I totally understand where you're coming from in asking this question. Here's the deal: you obviously understand that pushing yourself to run faster than is comfortable helps you become a better runner so I'm not going to focus on all the physiological benefits of pushing the pace. Speed work serves another function in addition to improving your fitness level: Speed work lets you practice mindfulness and intentionally choosing behavior.
What? Mindfulness at the track? Yes. When you're body is hurting and your brain is telling you it's time to quit, this is exactly the environment where psychological strength is built.
First let's tackle the "how do I make myself go do it" question. A simple answer, yet not always easy to execute: Just do it. Change your clothes, eat your banana and drink your coffee, put on your music and walk out the door. Your brain will be looking for all kinds of things you could do around the house to procrastinate. You might even find yourself wandering toward those tasks, but just recognize that it's happening and redirect yourself out the door. Just start running. You don't have to *feel* like running; just start running.
So now you're at the track and you're getting into the workout of the day. Now is when it gets interesting if you tune into the right things. Your body is sending your messages via sensations. What do you notice? How do your legs feel? What is your breathing like? What are your arms doing? How is your posture? Consider your stride length and turnover; should you alter these a little based on where you are in the workout?
Be conscious of the words you're using when you describe these sensations to yourself. Mindfulness practice uses neutral language. When you're pushing yourself, emotions get involved and your descriptions will reflect the emotions you're feeling, which strengthens the emotions. And if you're experiencing positive emotions, this is great, but if you're struggling and wanting to quit, that language is only going to make it harder to keep going. Emotions can't make you do something unless you let them. Strive to bring any negative language back toward neutral and notice how this can help you persevere.
In addition to noticing messages from the body, your brain is also talking to you constantly. What is it saying? Is it counting how many reps left to go? Is it analyzing your pace? Is it generating all sorts of excuses and reasons why it's okay to quit? Is it telling you that your goals are unattainable and you were crazy to think you could accomplish them? This is what brains do: They think. In practicing mindfulness, you practice noticing thoughts as simply thoughts. Thoughts come in and go out, unless we grab onto them and feed them with the emotions they are generating. Thoughts can't make you do something unless you let them. Noticing thoughts as thoughts helps keep them separate from emotions and lets you intentionally choose what you're going to do.
When you're at the track and practicing mindfulness like this, a watch comes in very handy. A watch will provide you with objective feedback about what is actually happening rather than relying on your perception of the effectiveness of the workout. Certainly there is a place for perceived exertion and "listening to your body" in working out and I'm not advocating for pushing through injury or ignoring signs of excessive fatigue. Sometimes you'll be surprised that what you thought was a total flop of a workout was actually a pretty decent performance after all.
Regardless of what the watch says though, and regardless of your thoughts and feelings during the workout, when you get to the end and were able to practice some mindful awareness (even for a few minutes), the workout was productive and the time was well spent. Good job, athlete. You win.