This article was originally posted by Ryan at the original HillRunner.com Blogs.
Note: This is an article I wrote many years ago without revision. It’s become somewhat popular so I want to preserve it as it first appeared as the articles section of the site gets phased out. In the future, I may write another post discussing how my views have evolved over the years but this generally is still advice I would stand by.
No matter where a runner looks, you can’t seem to get away from the term "junk miles". Don’t run too much on your easy days, you’ll be running junk miles. Don’t go that far on your cooldown, that’s just junk miles. Don’t do that extra run, that’s just junk miles. So, what exactly are junk miles and why are they so bad?
In my opinion, the term "junk miles" is the most overused term in running. Most people will use the term to describe any amount of miles that may leave you a little tired for a workout.
So, here is my definition of "junk miles": If you are doing so many miles that you can’t get in your speed workouts at a time when your speed workouts should be the focus, you are running junk miles. Of course, this is a pretty complex definition, so I better explain further.
During base training, when you are not running hard workouts or at the very least are not focusing on the hard workouts, is there such a thing as junk miles? As long as you are keeping yourself healthy, I argue no.
What about in the final couple of months of your training (few months, whenever your focus shifts), when your focus becomes the hard workouts? I think the best way to explain this is to use an example. Let’s say you have 4 mile repeats on the plan for Tuesday and 8×800 on the plan for Thursday. Your target paces are 5:20 and 2:30 (just using rough estimates of what my paces would be). You run your 5:20 miles on Tuesday, do 5 miles on Wednesday, then come back with your 2:30 800s on Thursday. Obviously, no junk miles there. Now, let’s say you are running higher miles. You end up doing 5:25 miles on Tuesday because you are a little fatigued after your 10 miles Monday, you do 10 miles Wednesday, then do the 800s in 2:32. Are the 10 mile runs on your easy days junk miles? Some people would say yes but I say no. If you are still getting the whole workout in at the goal intensity, hitting the exact paces isn’t crucial. Your body doesn’t even know it’s running 5:25 and 2:32 instead of 5:20 and 2:30, it just knows it’s running at the intensity that you wanted to run at in the first place. You are still getting in the desired training effect, plus you are building significantly more strength on the easy days, which will help you greatly on race day. However, let’s say you step it up another notch. You do your miles Tuesday but only get through 3 of them because you are so tired. You then do 15 miles on Wednesday, then only get through 5×800 on Thursday because you are again still tired. Are you now running junk miles? The obvious answer seems to be yes. My answer is maybe. If you are in a phase where building your aerobic strength is still most important, as it would be for quite some time if training for a longer race like a marathon, no. However, if you are in the final race preparation phase or the peaking phase, yes you have.
So, there is my term of junk miles. If you are doing so many miles that you can’t get in your speed workouts at a time when your speed workouts should be the focus, you are running junk miles. However, doing the workouts a little slower than planned because you are a little tired doesn’t mean anything. As long as you get the whole workout in at the desired effort.
Now that I went through this whole explanation, I’m going to throw a wrench in it. In most cases where I see people not getting through workouts, it’s not because they are running too many miles on their easy days. It’s because they are running too fast on their easy days, what you might call junk pace. There is no harm in running your easy days very slow. You will still build the aerobic systems that the aerobic runs are designed to build and you will recover more. In fact, the longer you are out there, the more work your body will do to build those aerobic systems. So, in many ways, an 80 minute run at 10 minutes per mile can be better than a 40 minute run at 8 minutes per mile.