Workouts based on pace are best done in controlled environments like an indoor track where there is no need to adjust for gradient, wind speed, temperature and the conditions underfoot. Most experienced runners are able to choose a pace based on their current level of fitness rather than where they would like to be.
Using one of the many pace charts online it is then possible to extrapolate an equivalent pace for different distances. These charts do not take into account a particular runners capacity for endurance running or ability to sprint. The predicted times are averages and don’t take into account an athletes genetic makeup in terms of abundance of fast or slow twitch muscle fibres or whether their training has been geared to a specific distance.
Since the majority of races are run in the real world it makes more sense to run your race at an effort level you can sustain to finish in the shortest possible time. This effort level will vary day by day and be influenced by training, rest, nutrition, the course profile, weather, etc. When running to effort you will instinctively slow down when you come across that surprise hill in the middle of the race. Running to a set pace will require a greater effort to hold that pace and you may never recover from that additional work.
That is not to say we should never use paces in a workout but they should be used as a guideline and not rigidly adhered to. Workout paces will tell you your level of fitness and show your training progress. However it is more useful to review the execution of your workout when you get home and ask yourself. Was I consistent during a set? Was the interval pace one that could have been maintained in race conditions? How do these numbers compare to similar workouts done earlier in the season? What external factors (sleep, work, rest, etc.) may have influenced the training session?
Being intuitive, running by effort and getting a sense of your pace whenever you run, without reference to your watch, will help you to become the best runner you can be.