The long run is the most important workout in any distance runner's week. It is also the workout most runners execute incorrectly — not by skipping it, but by running it wrong.
The error is almost always the same: running it too fast.
I spent two marathon cycles running my long runs 30–40 seconds per kilometre faster than I should have. They felt productive. They were not. I arrived at each long run already fatigued from the last, my easy runs got compromised, and my CTL plateaued because I couldn't absorb the training load. The fix was humbling: slow the long run down by 40 seconds/km and it stopped being a race against myself and started being the aerobic stimulus it was designed to be.
What the long run actually does
The long run develops the physiological systems that make distance running possible:
- Mitochondrial density — more mitochondria means more aerobic energy production per cell
- Capillary development — denser capillary networks deliver more oxygen to working muscle
- Fat oxidation — your body becomes more efficient at burning fat, sparing limited glycogen
- Glycogen storage capacity — repeated depletion trains the muscles to store more
- Musculoskeletal durability — tendons, ligaments, and bones adapt to sustained load
- Mental resilience — practising sustained effort builds the psychological tolerance racing requires
Critically, every one of these adaptations is driven by duration at low intensity — not by speed. This is why pace discipline matters so much.
The number one mistake: running too fast
Most runners run their long runs at moderate intensity — somewhere between genuine easy pace and marathon pace. It feels right. It feels like training. It is the single most common error in distance running.
Here is the problem. A long run at the correct E pace produces aerobic adaptation while keeping the recovery cost manageable. A long run 30 seconds/km too fast produces marginally more aerobic stimulus but a disproportionately larger increase in fatigue (ATL). That extra fatigue then compromises the next two or three days of training.
The result is a runner who is always slightly tired, whose easy runs are never truly easy, and whose quality sessions suffer because they are run on fatigued legs. The long run that felt productive quietly degraded the whole week.
The rule: Your long run should be run at E pace — conversational, relaxed, the pace at which you could comfortably hold a conversation. If you finish a long run feeling like you raced it, you ran it wrong.
How far should your long run be?
Two principles govern long-run distance:
Principle 1: 20–30% of weekly volume. Your long run should be roughly a quarter to a third of your total weekly mileage. A runner at 50 km/week does a 13–16 km long run. A runner at 80 km/week does a 20–26 km long run. This keeps the long run proportionate — long enough to stimulate, not so long it dominates and damages.
Principle 2: The 2.5-hour ceiling. Jack Daniels is explicit that long runs beyond approximately 2.5 hours produce diminishing aerobic returns while the injury and fatigue cost keeps climbing. For most runners this caps the long run well before "marathon distance" — and that is intentional. You do not need to run 42 km in training to race 42 km.
Practical distance targets:
| Race | Long Run Peak | Frequency |
|---|---|---|
| 10K | 14–18 km | Weekly |
| Half marathon | 18–22 km | Weekly |
| Marathon | 29–32 km | Every 7–10 days at peak |
Marathon long runs of 32 km are appropriate at the peak of a 16-week marathon plan. Half marathon long runs rarely need to exceed 22 km — covered in detail in the half marathon training plan.
Easy long runs vs. quality long runs
Not all long runs are the same. There are two distinct types, and using them at the wrong time is a common periodization error.
The easy long run is the default. Run entirely at E pace. Its job is pure aerobic development and durability. This is what you do in the base phase and on most weeks of any training block.
The quality long run incorporates faster segments — typically marathon-pace (M pace) miles in the final third of the run. Example: a 29 km long run where the last 10 km are run at M pace. This teaches your body to hold race pace on tired legs and trains the specific metabolic demands of racing. It belongs in the build and peak phases of periodized training, not in the base phase.
The mistake is doing quality long runs too early, or doing them every week. A quality long run carries a much higher recovery cost. One every 2–3 weeks during the build phase is sufficient. The rest should be easy.
Fueling the long run
For long runs under 75–90 minutes, fueling is optional — your glycogen stores are sufficient.
Beyond 90 minutes, fueling matters. Muscle glycogen depletes meaningfully around the 75–90 minute mark. Running beyond that without carbohydrate forces heavy reliance on fat oxidation, which slows you down and spikes perceived effort — the "running on empty" feeling.
Practical fueling guidance:
- For runs over 90 minutes, take 30–60g of carbohydrate per hour
- Start fueling early — around 45–60 minutes in — not when you already feel depleted
- Use your long runs to practise race-day fueling. Your gut needs to adapt to absorbing carbohydrate while running. A runner who never fuels in training and then takes three gels on race day often suffers GI distress
- Hydrate to thirst; in heat, run by heart rate rather than pace as cardiac drift will push your HR up
How often should you do a long run?
For most intermediate runners, one long run per week is the standard. It typically falls on the weekend when time allows.
During heavy marathon training, the longest long runs (29–32 km) carry enough recovery cost that they are best done every 7–10 days rather than strictly weekly — alternating with a slightly shorter long run. This is where Pfitzinger's medium-long run concept comes in, covered in the Pfitzinger vs Daniels comparison.
What matters is consistency over months. A single 30 km run does little. Twelve progressively building long runs across a training block builds a marathon.
Long run by training phase
The long run is not static — it evolves across a periodized training block:
- Base phase: All easy long runs. Gradually building distance. Pure aerobic development.
- Build phase: Distance near peak. Introduce quality long runs with M-pace segments every 2–3 weeks.
- Peak phase: Longest long run of the cycle, often with the largest M-pace block. Highest recovery cost.
- Taper: Long runs shorten sharply. The final long run is short and entirely easy — covered in the taper guide.
Why adaptive plans get the long run right
A static plan prescribes the same long run progression for every runner regardless of how they are absorbing the training. If you arrive at week 11 with a lower CTL than the plan assumed — because of illness or missed sessions — a 32 km long run at the prescribed pace may be too much, spiking ATL beyond what you can recover from.
Adaptive training plans scale the long run to your actual state. If your recent training load is lower than planned, the long run is shortened or the M-pace segment reduced. If you are absorbing load well, it progresses as scheduled. The long run adapts to the runner — not the other way around.
The long run works because of what it is: sustained aerobic effort at the right intensity, repeated consistently over months. Run it easy, build it gradually, fuel it properly, and it becomes the foundation everything else is built on. Run it too fast, and it quietly undermines the very fitness it is supposed to build.
Frequently Asked Questions
How long should a long run be?
A long run should be roughly 20–30% of your total weekly volume, and rarely exceed 2.5 hours regardless of distance. For a runner doing 60 km/week, that means a long run of 18–22 km. For marathon training, long runs peak at 29–32 km; for half marathon training, 18–22 km. Going beyond 2.5 hours produces diminishing aerobic returns while sharply increasing fatigue and injury risk.
What pace should I run my long run at?
Most long runs should be run at easy (E) pace — genuinely conversational, roughly 60–75% of max heart rate. This is the single most important and most violated rule of long-run training. The aerobic adaptations come from time on feet at low intensity, not from speed. Quality long runs that include marathon-pace segments are a separate, more advanced session used in specific training phases.
Should I eat during a long run?
For runs over 90 minutes, yes. Muscle glycogen stores begin to deplete meaningfully around the 75–90 minute mark, and running beyond that without fueling forces your body to rely heavily on fat oxidation, slowing you down and increasing perceived effort. Practice race-day fueling — 30–60g of carbohydrate per hour — during your long runs so your gut adapts before race day.