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You’ve committed to the marathon. The plan is printed (or saved to your phone), the long runs are blocked out in the calendar, and you’re genuinely motivated. Then around week 10 or 12, something shifts. Your legs feel heavy on runs that should feel easy. You’re sleeping eight hours and waking up tired. Your pace is slower, not faster, despite all the work you’ve been putting in.
That’s not weakness. That’s overtraining — and it catches more marathon runners than you’d think, especially those who are too motivated. The runners who skip rest days, add extra miles “just because”, and treat every session like a test.
This article won’t tell you to “listen to your body” and leave it there. You’ll get specific signs to watch for, practical rules to follow, and a clear framework for training hard enough to improve without grinding yourself into injury or burnout.
What overtraining actually is (and what it isn’t)
Overtraining isn’t just feeling tired after a long run. Post-run fatigue is normal and expected — that’s the stress that drives adaptation. Overtraining, more accurately called Overtraining Syndrome (OTS), is what happens when you accumulate training stress faster than your body can recover from it, over a sustained period.
The tricky part is that the stage before full OTS — sometimes called “functional overreaching” — is actually where some of your best fitness gains happen. Push a little beyond your comfort zone, recover properly, and you come back stronger. The problem is when you stay in that overreached state for too long without adequate recovery. Days of accumulated fatigue become weeks, and performance starts to drop rather than rise.
For everyday marathon runners juggling jobs, families and imperfect sleep, the recovery half of that equation is almost always the weaker side.
The warning signs that are easy to miss
Some overtraining signs are obvious. Persistent injury, complete exhaustion, dreading every run. But the earlier signals are subtler, and catching them early is what keeps your training on track.
Watch for these:
- Easy runs feeling genuinely hard. If your usual 5:50/km easy pace now feels like a tempo effort, that’s a red flag — not a reason to push harder.
- Elevated resting heart rate. A resting HR that’s 7–10 bpm above your normal baseline, consistently over several days, is a reliable early indicator. This is worth tracking.
- Sleep disruption despite fatigue. Overtraining affects the autonomic nervous system, which can cause poor sleep quality even when you’re exhausted.
- Mood changes. Irritability, low motivation, and a flat feeling that isn’t just a bad day are documented symptoms of overtraining, not just a mindset problem.
- Stale performance. If your 5K time trial or tempo run paces aren’t budging despite months of training, your body may be too fatigued to adapt.
The research from the European Journal of Sport Science consistently flags mood disturbance and performance plateau as the most reliable markers — more so than any single physical symptom.
The most common overtraining mistakes in marathon prep
Most runners don’t overtrain by doing one catastrophic thing. It creeps up through a collection of small, well-intentioned decisions.
Increasing mileage too fast. The 10% rule — don’t increase weekly mileage by more than 10% from one week to the next — is a rough guideline, not a law of physics. But it exists for a reason. Going from 40km to 55km in a week because you had a good stretch is how niggles start.
Skipping the easy days. If your plan says easy 8km on Wednesday, running it at 5:10/km because you feel good isn’t a bonus. It’s eating into your recovery for Thursday’s quality session.
Treating every long run like a time trial. Your long run should feel controlled and conversational — aim to hold a pace roughly 60–90 seconds per km slower than your goal marathon pace. If your marathon target is 5:00/km (a 3:30 finish), your long runs should sit around 5:45–6:00/km for most of the distance.
Not building in cutback weeks. A good marathon plan reduces mileage by roughly 20–25% every third or fourth week. If your plan doesn’t have these, add them yourself.
Training through illness. If you have a fever or symptoms below the neck (chest, stomach), rest. The “neck rule” from sports medicine is imperfect, but it’s a reasonable starting point for a decision that too many runners get wrong.
How to structure your training to prevent it
The most effective prevention is building recovery into your plan from the start, not bolting it on when things go wrong.
| Training element | What it should look like | Common mistake |
|---|---|---|
| Easy runs | 65–70% of max HR, can hold a full conversation | Running at “comfortable” but still pushing |
| Long run pace | 60–90 sec/km slower than goal marathon pace | Racing the long run |
| Weekly mileage increase | Max 10% per week, with cutback every 3–4 weeks | Building continuously for 6–8 weeks |
| Cutback weeks | Drop volume by 20–25%, keep some intensity | Skipping them when training feels good |
| Sleep | 7–9 hours — more important in heavy training blocks | Treating 5–6 hours as manageable |
| Rest/cross-training days | At least 1 full rest day per week | Replacing rest with “easy” runs |
One practical tool worth using: if you wear a GPS watch with heart rate, check your resting HR each morning before getting up. Apps like Garmin Connect and Polar track this automatically. A rising trend over 5–7 days, without an obvious explanation like illness, is worth cutting your next session shorter or easier.
How many days a week should you actually be running?
For most everyday marathon runners — those training 12–20 weeks for their first or second marathon — three to five days of running per week is the realistic sweet spot. Five days works well if your life allows for genuine recovery between sessions and you’re sleeping reasonably well. Three days works if you’re time-pressed, provided those three runs include a long run, one quality session, and one easy run.
The runners most at risk of overtraining are often those jumping from three to six days a week because a plan tells them to, without building the base gradually. If you’ve been running four days a week for a 10K block, don’t launch immediately into a six-day marathon schedule.
The NHS physical activity guidelines note that rest and recovery are part of a training programme, not gaps in it — a framing worth keeping in mind when you feel guilty for taking a day off.
What to do if you think you’re already overtrained
First: don’t panic and don’t compensate by training harder. That instinct to “run through it” is almost always wrong.
If you’ve had one or two rough weeks, a 5–7 day period of easy running only (no intervals, no tempo, no long effort) is usually enough to reverse early-stage overreaching. Keep the runs short — 30–45 minutes at genuinely easy effort.
If you’ve been feeling flat for three weeks or more, seriously consider a full week off. Yes, you’ll feel like you’re losing fitness. You won’t lose meaningful fitness in a week. What you will do is give your nervous system and hormonal system time to reset.
After any recovery period, rebuild at 70–80% of the mileage you were doing before you backed off. Don’t try to “make up” lost sessions. The miles you missed are gone — chasing them is how you go straight back into the same hole.
If symptoms persist beyond two weeks of reduced load — including mood changes, sleep problems, and sustained performance decline — it’s worth speaking to a GP or sports medicine doctor. Overtraining Syndrome at a clinical level is relatively rare, but it is real and it does take months to resolve.
The role of stress outside running
This part gets skipped in most training articles, but it matters: your body doesn’t separate running stress from work stress, poor sleep, or a difficult week at home. Cortisol is cortisol. If you’ve had three terrible nights of sleep and a high-pressure week at work, doing your hardest tempo session of the plan on Friday is not the same workout it would be in a well-rested, lower-stress week.
This doesn’t mean cancelling every hard session when life gets busy. It means being honest about what your body is actually dealing with across the whole picture. Drop the session to an easy run sometimes. Shorten the long run by 4–5km when you’re running on empty. The training block is 16 weeks — one adjusted session won’t derail it, but ignoring accumulated life stress for six weeks straight might.
The honest takeaway
- Tired legs after hard sessions are normal. Tired legs on every run, including easy ones, is a warning sign — cut back mileage for 5–7 days and see if things change.
- Build cutback weeks into your plan from day one, not just when you’re struggling. A 20–25% volume drop every 3–4 weeks is standard, not optional.
- Your long run pace should feel genuinely manageable — if you’re targeting a 4:30 marathon (6:23/km), your long runs should mostly sit around 7:00–7:15/km.
- Resting heart rate is a cheap, practical monitoring tool. A consistent elevation of 7–10 bpm over several days is a signal worth acting on, not pushing through.
- You cannot out-train a recovery deficit. The adaptation happens during rest. More miles without more recovery doesn’t make you fitter — it makes you slower and more injury-prone.
Next read: Struggling to structure your marathon training weeks? Read our guide to building a marathon training plan that works around real life → /marathon-training-plan-for-beginners