The stress and recover principle is at the heart of our training as distance runners. The purpose of a stress workout is to stress certain systems of the body in such a way as to cause desired adaptations to take place. In other words to become fitter in a desired way. The purpose of a recovery run is to allow the body to recover from a stress workout while maintaining or advancing cardiovascular fitness.
Note that I never say the word pace or time in either definition.
I love math more than most, and am an analyst by nature, and so I have fun working through all the formulas and determining the optimal paces for workouts and runs based on the science behind the sport. But sometimes knowing the theoretical optimal pace does not necessarily help us, and in fact can hurt us by putting extra pressure on us that can detract from our performance in the workout or race.
I suggest the best way to do any run is to start off and find the rhythm and effort level that feels right for that type of run/workout. Then after a little while, once you have settled in, check your watch as a feedback tool to verify you are somewhere in the general desired range or where you thought you were.
There are many factors that can effect the optimal pace range on any given day, so by having a good feel for and knowing the rhythm of a desired workout, you can often be better assured of meeting the goal of the day, whether it be to stress the body or allow it to recover. Given this I am in the process of making a change as a coach and including more talk about the desired feel of a workout in my notes to my runners and only including pace ranges as a secondary note that they can use as a feedback tool, not as a driver of the workout.
What you want to stay away from is being obsessed with your watch. Checking your time/pace multiple times per mile is not healthy mentally or physically. Hitting a certain time/pace does not assure the optimal outcome for the run, and it can be a dangerous crutch that can be disastrous if ever it is not available or if it malfunctions (which happens often).
Keep your eye on the goal of run, and no that does not usually mean a specific time/pace, but rather honoring the stress and recover principle. If your workout for the day is a 5 mile tempo run, and you go out and run it at a good hard, smooth effort, working hard but keeping it controlled and manageable, you will have gotten the desired benefits from that workout regardless of what your watch says. Similarly, if you have an easy 5 mile recovery run planned in order to recover from the stress workout you did the day or 2 before, and you go out and keep it comfortable and relaxed and never feel like you are pushing it then you will have accomplished the purpose of that run, regardless of what your watch says.
What you use the watch for on these runs is feedback tools to help you fine tune and better understand those efforts and feel over time, and to monitor and track progress and to adapt the training plan. It needs to be a tool to enhance training, not a driver that dictates it.