If I could fix one habit in most self-coached runners, it would be this: run your easy days easier, and your hard days properly hard. It sounds almost too obvious to bother writing down. It's also one of the biggest differences between training that works and training that quietly grinds you down. And I say that as someone who once tried to run seven days and 100km a week as a 40-year-old. Spoiler alert: not one of the experts' recommended methods. Who knew.
To get it right you need to properly understand one word, threshold, and stop guessing at another, "easy". Let's take them in turn.
Threshold, in plain English
Your threshold pace is, roughly, the fastest pace you could hold for about an hour. It's "comfortably hard": you're working, you could gasp out a few words but not hold a conversation, and you can feel you're right at the edge of what you could keep up. Nudge the pace up even slightly and it would soon start to fall apart.
Physically, it's the point where your body starts producing fatigue quicker than it can clear it. Train near that line and the line itself moves. That's the good bit. You become able to hold a strong pace for longer before it bites. It's why threshold work is the most productive running most of us can do: a big return for the effort, without the wreckage that all-out sessions leave behind.
The mistake: living in the middle
Without a plan, here's what tends to happen. You head out, settle into a pace that feels "normal", and it lands somewhere in the middle. A bit too hard to be easy, a bit too easy to be a proper workout. Do that most days and you get the worst of both worlds: enough fatigue to wear you down, nowhere near enough stimulus to make you faster. It's the no-man's-land of running, and it's where an awful lot of people quietly live. Comfortable? Not really. Useful? Also no.
The fix: 80/20
The approach that keeps winning out in the research is simple to say: around 80% of your running should be genuinely easy, and the other 20% properly hard, threshold or faster. Not a permanent grey smear in between.
Easy days easy, so the hard days can be hard.
And before you dismiss this as the latest training gimmick, it isn't. It's one of the better-evidenced ideas in endurance running. When Stephen Seiler measured what the world's best endurance athletes actually do, they weren't grinding away in the middle: they spent roughly 80% of their training easy and the rest properly hard (Seiler & Kjerland, 2006). "That's the elites, though," I hear you say, and fair enough. So when researchers took ordinary recreational runners and put one group on an 80/20 plan and another on a threshold-heavy one, the 80/20 group came out faster, trimming around 5% off their 10K against 3.6% for the others (Muñoz et al., 2014). And in a straight shoot-out between four different approaches in trained athletes, polarised won on most of the markers that matter (Stöggl & Sperlich, 2014). Not a fad. The actual evidence.
"Genuinely easy" is the part everyone resists. Easy means conversational, full sentences, not gasped fragments. If you can't chat, you're not running easy, you're running in the grey zone. For most people that pace feels almost embarrassingly slow at first, and every instinct screams that it can't possibly be doing anything. Trust it. It's where your aerobic engine is quietly built, and it's the thing that lets you absorb the hard sessions that move the needle.
How to find your numbers
The good news is you don't have to guess. Pop a recent race or time-trial result into the race pace calculator and it'll hand you your easy, threshold and interval paces from your current fitness, not a one-size-fits-nobody chart. Treat the easy number as a ceiling, not a target: slower is absolutely fine, faster quietly defeats the whole point.
Get that right and the week more or less runs itself: most of it easy, one or two sessions at threshold, and you're suddenly training the way the science has been politely suggesting all along, instead of grinding out every run in no-man's-land and wondering why the times won't budge.