Here’s Why Your Easy Runs Slow Down During Marathon Training

Is something wrong if your easy runs are getting slower during marathon training – or is this a normal response to peak weeks?

I receive this question frequently from my athletes. Runners training for their first marathon often are the most alarmed, but experienced marathoners are not immune to this worry during training.

The concern is understandable. You invest time, energy, and often money into training for a marathon (or half marathon, ultra, etc.). You want to be getting faster after those months of hard work, not slower.

Fatigue Temporarily Masks Fitness

While there are outlier scenarios – medical conditions and overtraining – the general answer is that you are in a temporary “valley of fatigue.” It’s not that you are getting slower, it’s that you are completing your easy runs with more accumulated fatigue than normal.

The supercompensation theory in exercise science provides a framework for understanding how fatigue masks performance.

In this theory, you have a fitness baseline, which represents your starting point at the beginning of your marathon training block. Over the course of the 12-18 weeks that you train for your race, you progress your training with longer runs, greater weekly volume, and progressively more challenging workouts. If it is your first marathon, you may be completing the longest run you’ve ever done every week or two.

These training stimuli produce fatigue. They have to – that’s how your body adapts. However, that fatigue doesn’t automatically resolve within the next 24 hours before your next run. Depending on variables such as sleep and nutrition, it may take 48-72 hours to fully recover from a long run or hard workout.

You Do Not Fully Recover Between Runs (Even When Recovering Well)

A recent study in the Journal of Physiology demonstrates that even large amounts of carbohydrates (10 g/kg of bodyweight!) do not fully replenish muscle glycogen within 12 hours after two hours of endurance training. As someone who worked through athlete nutrition assessments for practicums in graduate school, I can assure you that the majority of recreational runners do not refuel with that high a dose of carbohydrates.

Given that, we can assume that most runners take 24+ hours to replenish their glycogen stores – and that’s only one aspect of recovery. Central and peripheral system fatigue and muscle damage mean it may take a couple of days before you feel fresh.

In a smart training plan, workouts are programmed accordingly, with 2-4 days between long runs and key workouts. In between those days, you typically have easy runs – and therefore those easy days are the most impacted by fatigue.

While you don’t want excessive levels of fatigue – too much training load will lead to maladaptation – you will experience mild fatigue as you progress your training. If you look back at the supercompensation curve, you see where fatigue temporarily yet noticeably dampens your performance during training.

As supported by a 2017 study in the International Journal of Sports Medicine, deliberate overreaching will be reflected in your submaximal heart rate, rate of perceived exertion (RPE), and exercise performance. Whether you use heart rate or RPE on your easy runs, this fatigue will result in a slower pace during the heavy weeks of training.

However, Easy Runs Should Not Be a Struggle

It is worth noticing that training fatigue should be mild. You shouldn’t be exhausted by mid-day, tired upon waking, experiencing mood changes, loss of motivation or struggling to complete most runs. Those are signs of under-recovery and maladaptation; if you continue to push through, you risk developing overtraining syndrome. (These signs and symptoms can also indicate medical issues such as thyroid dysfunction, poor iron status, relative energy deficiency in sport, and other medical issues, so never hesitate to have bloodwork done if symptoms persist.)

Mild and normal training fatigue presents as feeling a bit less peppy and a bit slower on easy days and recovery runs. You may need extra sleep at night, but you should wake up able to handle the cognitive and physical demands of your day. You should still be able to complete most of your training runs.

Fatigue’s suppression of performance is only temporary. As shown in the supercompensation curve, the deliberate reduction of training improves performance, which is why you taper before a goal race.

When you taper, you remove the training stimulus and therefore reduce fatigue. As a result, two to three weeks of taper produce the supercompensation effort. You recover from training and your fitness improves above your original baseline. That spike in fitness is considered peak performance.

In summary, it’s very common for easy runs to be slower in marathon training. Do not force a pace and make the mistake of running easy days too hard – and accumulating excessive fatigue. Too many runners are still fatigued on race day due to too fast of a pace on easy runs. Instead, focus on RPE or heart rate. Let those runs slow down so you can recover for the long runs and hard workouts.

Additionally, pay attention to how you feel on long runs and hard workouts. If you consistently aren’t recovered in time for those, then you may need to scale your training load or improve your recovery habits, such as sleep and nutrition.

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