Why the Long Run Still Matters: The Science Behind Your Weekly Non-Negotiable

If you're building a training plan and looking for the one session to cut when life gets busy, let me save you the trouble: it's not the long run.

I get asked about this constantly. Athletes will happily double up on speed work, but the long run is the first thing to go when the week gets tight. That's backwards. The long run isn't just “more mileage” — it's doing physiological work that no other session on your calendar can replicate. And it's not just something you need if you are training for the marathon. From the 1500m event and upwards, the Long Run is one of the most important sessions of the week.

Here's what's actually happening in your body during those long efforts, and why skipping them costs you more than you think.

What the Long Run Is Actually Building

Mitochondrial density. Sustained aerobic effort is the single strongest stimulus for increasing the number and efficiency of mitochondria in your muscle fibres. More mitochondria means more capacity to produce energy aerobically — which means you can hold a faster pace before you tip into anaerobic metabolism and start accumulating fatigue.

Capillarisation. Long, steady efforts drive the growth of new capillaries around your muscle fibres. More capillaries means more surface area to deliver oxygen and clear metabolic waste. This is a slow adaptation — it takes consistent time under aerobic load, not a single hard interval session, to build.

Fat oxidation and glycogen sparing. The longer you run, the more your body is forced to learn how to use fat as fuel and preserve glycogen. This is trainable. Athletes who consistently skip long runs tend to burn through glycogen faster at a given pace, which is exactly why they “hit the wall” earlier in races.

Cardiac output. Time spent at sustained aerobic effort increases stroke volume — the amount of blood your heart pumps per beat. A stronger, more efficient heart lowers your resting and submaximal heart rate and improves oxygen delivery across the board. This adaptation responds specifically to duration, not just intensity.

Musculoskeletal durability. Long runs condition tendons, ligaments, and connective tissue to tolerate repeated loading over time. This is why athletes who build up long run duration gradually are far more resilient on race day than those who rely on shorter sessions and then jump straight into race distance.

Fatigue resistance and running economy under fatigue. This is the one people underestimate most. Your running form when fresh isn't the form that decides your race — it's your form at kilometre 30, or in the last 10K of a marathon, when everything hurts and your brain is looking for an excuse to slow down. The long run is the only session that trains you to hold technique, pacing discipline, and mental focus while fatigued. There's no substitute for it.

Why Skipping Your Long Run Hurts More Than You'd Expect

Miss a threshold session and you lose a bit of sharpness — recoverable in a week or two. Consistently miss long runs and you're undermining the base the rest of your training sits on. Speed and threshold work make you faster over a distance you're aerobically prepared for. The long run is what extends that distance, builds the fuel efficiency to sustain your pace, and prepares your body — physically and mentally — to hold together when it counts.

Athletes who under-do their long runs typically see the same pattern in racing: solid pace through halfway, then a disproportionate fade in the back third. That's not a mental toughness problem. That's a training gap.

Tangible Takeaways You Can Apply This Week

1. Protect it like a race, not a “should.” If your week is tight, cut volume from an easy midweek run before you cut the long run. The adaptations above are duration-dependent — you can't compress them into a shorter session.

2. Build gradually — the rough 10% rule is a floor, not a target. Increase long run duration by roughly 10% per week until you get to roughly 90-100 mins of running in one go. From there you want to hold that volume in training weeks before tpaering your volume down for races.

3. Practice race-day fuelling on every long run over 90 minutes. Your gut is trainable. If you're racing anything over 90 minutes, your long runs are where you rehearse exactly what and when you'll eat and drink — not race day itself.

4. Keep most long runs conversational. The aerobic and mitochondrial adaptations above come from time at moderate, sustainable effort — not from turning every long run into a time trial. Save the harder long run efforts (progression runs, race-pace finishes) for when they're specifically programmed.

5. Treat the final third of the run as the point. If you're cutting long runs short when you get tired, you're skipping the exact part of the session doing the most work. That's where fatigue-resistance and mental toughness are built.

6. Recover with the same intensity you trained with. Long runs create a bigger fatigue cost than any other aerobic session. Prioritise sleep, protein intake, and an easy or rest day immediately after — don't stack quality sessions the next day.

The Bottom Line

The long run isn't filler mileage. It's the session most responsible for the aerobic engine, fuel efficiency, and durability that everything else in your plan is built on top of. Treat it as non-negotiable, build it up patiently, and use it deliberately — and you'll feel the difference exactly where it matters: late in the race, when everyone else is fading.

This blog was written for the PGC1-Coaching website by Joshua Schofield. Josh is the Head Coach and Founder of PGC1-Coaching. With a background as a sport and exercise scientist and former international athlete; Josh has developed a deep understanding of sports science and endurance coaching. Josh leads the PGC1 team with a passion for helping athletes of all abilities reach their potential. His coaching philosophy blends evidence-based training with a personalised, athlete-first approach.