You have to get to know your body! You should be able to look at the whiteboard and have a realistic estimate of how long each part of the workout will take you. This is not cheating or giving less than your best, it’s enabling you to create a plan; it is what every good athlete does. You have to set a pace and hold the pace to continue to progress. If you do the same thing every workout you will get the same result.
If you go out hot every workout and end up burning out and taking lots of rest in the middle of the workout your pace is not sustainable. You are not getting the most out of your workout. Learning to control the pace is essential to your performance. Controlling the pace does not only set you up for success, it enables you to control the lactate build-up in your system. This lactate build-up is what causes your muscles to ache and stop working.
As you get fitter you will be able to flush out your system faster and more efficiently which will make you faster. Again by doing the same thing every time; going out hard, getting tired, slowing down, or resting, you are not training your body to flush out anything. The same goes for the opposite if you never push and you play it safe in workouts you will never find the next level. Some days you have to go for it and you may fail, but at least you tried and now next time you have a pace to go off.
Mistakes are where we learn about our pacing and our threshold. Creating a plan for yourself is a great first step. The next step is to analyze that plan- did it work? Did it not work? Why did it not work? What would you change? Reflecting on your performance is the best and easiest way to start learning your abilities and becoming self-aware.