When I was in college, my coach had a term for people who would run their threshold runs harder than he asked: “Threshold All Americans.” It was meant as a derogatory term or, more gently, as a phrase to try to discourage them from going so hard.
But, if they were feeling good and the workout felt easy, why not run faster? You get more out of it, right?
No. The workout was intentionally designed to be relatively easy. Either due to higher load workouts around it or simply because he wanted us to be more fresh for an upcoming race or another workout, our coach wanted us to do something that was a little easier on that day. However, our “Threshold All Americans” didn’t get it. They thought every workout should be hard and they insisted on making it that way.
When you run a workout other than designed, even if it feels too easy as designed, you’re not getting the most out of your training. Yes, something like “4 miles at half marathon effort” may not feel like much of a workout but your coach or the person who wrote your training plan knows this and planned your training this way for a reason. Maybe because you need to be fresh for a big workout or long run later in the week. Maybe because you have a long time until your goal race and the risk of burnout from doing very hard workouts for too long is high.
Of course, this doesn’t mean you don’t adjust when appropriate. Maybe, if the conditions are perfect and you’re feeling great, you can go a little faster but just a little. If the conditions are bad, you should adjust your paces to account for the conditions. If you’re feeling a little run down, a lot of considerations go into the decision but one option is to allow the pace to be slower. This is far different than just running much faster than asked just because it feels “too easy”, though.
Remember, every workout was designed the way it was for a good reason. If you want to understand the reason, then ask. But, whatever you do, don’t become a Workout All American.