The Grey Zone: Why some Runners Get Threshold training Wrong

You’ve heard it before: “run your tempos at threshold.” Simple advice, right? But run too fast, too long, or too often, and you end up in the dreaded grey zone; tired, flat, and not getting fitter. Here’s how to fix it and make your threshold work actually work.

The danger of generic threshold training

Threshold training is often sold as the golden ticket to endurance fitness. You see it on training plans everywhere: 20-minute tempo runs, 4 x 1 mile “cruise intervals,” or 40 minutes at comfortably hard pace. The concept is sound. The problem is that these workouts don’t mean the same thing for every runner. Jack Daniels, Steve Magness, and Matt Fitzgerald (3 coaches that I've read a lot from) all agree on one key truth: threshold training is powerful, but only if it’s specific to you. 

Too many runners fall foul of training in the wrong zones. They follow generic workouts without understanding the why, and end up working too hard, to recover properly or too easily to improve.

Getting threshold right is about relative precision, not perfection. It’s about matching intensity, duration, and progression to your event and your current fitness level and realistic goals.

You have more than one threshold

Most runners think of threshold as a single pace. Tempo running, right? It’s not. In reality, there are several thresholds, and each one serves a different purpose.

• Aerobic threshold (AeT): The point where your body starts to rely more on carbohydrates than fat for fuel. Training around here improves aerobic efficiency.

• Lactate threshold (LT): The effort where lactate builds up in the blood faster than it can be cleared. This is the “comfortably hard” zone most threshold sessions target.

• Anaerobic threshold: A sharper, higher-intensity point that suits shorter-distance racing and speed-endurance development.

So, If your training sits just below your lactate threshold, you work endurance and strength. Slightly above it, you work more on speed and tolerance. Both are useful, but each serves a different race goal, and importantly the duration, frequency and intensity needs adjusted with each context. Make sure that when you're doing threshold work that it's the right type for you, and that it is within a plan that focusses on progression and specificity.

Too fast, too slow, too much, too soon...

Running too hard during threshold work is one of the most common mistakes I see. I sometimes do it myself on occasions if "I feel good" or I get competitive with myself on certain days. It feels productive because it’s tough, but it’s often misplaced intensity. Instead of improving aerobic capacity, you’re dipping into anaerobic work and creating fatigue that might lingers for days.

On the flip side, running too slow means you never challenge the system enough to trigger adaptation. You feel comfortable, but you’re not pushing the physiological edge that makes you stronger. Matt Fitzgerald, in Run Faster from the 5K to Marathon, calls this the “grey zone trap”, that murky middle ground where you’re working hard enough to feel some fatigue but not hard enough to drive real progress. He stresses that every threshold session should have a clear purpose: are you developing endurance, strength, or speed? If you can’t answer that question, you’re probably in the grey zone.

match thresholds to your goals

If you’re training for a 5K or 10K, in general, your threshold runs should sit slightly faster, with shorter reps. Think 4 to 8 minutes of effort with short recoveries. This sharpens your lactate tolerance and helps you sustain faster race pace. For half marathoners, the classic 20-minute tempo or 3 x 10 minutes at threshold works beautifully. It builds aerobic power and teaches your body to stay strong under steady pressure.

For marathoners, the emphasis should shift slightly down, with some sprinklings of typical half marathon tempo efforts. Your threshold sits a touch below lactate level, blending into marathon pace. Workouts might look like 2 x 20 minutes at steady effort, or segments of threshold running built into your long run. Daniels’ rule of thumb still applies: keep threshold work to around 8 percent of your weekly mileage and aim for once a week. The key is not volume, but progression.

Progression over repetition

Threshold training isn’t about repeating the same weekly session week after week. It’s about building the challenge as your fitness develops. If you start your marathon block with 20 minutes at threshold, you might progress to 2 x 15 minutes, then 3 x 10 minutes, then a 40-minute continuous effort. Eventually, you might include those efforts inside a long run or near race pace, often in addition to a dedicated threshold workout.

As Matt puts it, consistency doesn’t mean doing the same session over and over. It means doing the right work consistently, for where you are in your training cycle. That’s how you create adaptation, not stagnation.

Your next step

If you’ve been doing 'tempos' for a while, take a step back and assess whether your sessions truly fit your goals. Are they the right length and intensity for your race? Are they building, or just repeating? Get your threshold right, for what you need to develop for your key goals, and you’ll unlock some of the most powerful, efficient gains in running. Get it wrong, and you will likely spend a lot of time in the grey zone; tired, flat, and wondering why all that hard work isn’t paying off. If you need some support to help you get threshold right for you, don't hesitate to reach out to see how I can help.

References

• Daniels, J. (1992). Cruise Control – Training Breakthroughs for Running. Runner’s World.

• Magness, S. (2014). The Science of Running. George Mason University.

• Fitzgerald, M. (2012). Run Faster from the 5K to the Marathon: How to Be Your Own Best Coach. Da Capo Press.