In The Long Run 

The long run should have it’s place in everyone’s training schedule. No matter the distance you’re training for or your experience level, the long run plays a key role in building endurance, resilience, and mental toughness that underpin all your training. It’s not just a “long jog” it’s a deliberate, structured stimulus that teaches your body how to cope with fatigue, improves your aerobic efficiency, and prepares you for the unique demands of running for an extended period. Just like other areas of training, the long run is a key part for  producing adaptations that may not be obvious day to day, but pay off when it matters most on race day.

Running for extended periods of time, particularly beyond 90 minutes, stimulates a series of physiological adaptations that make your aerobic system more efficient. One key change is an increase in the number and efficiency of mitochondria, these are seen as the tiny ‘powerhouse’ in your muscle cells that generate energy. As the density of mitochondria increases, your muscles become more efficient at producing energy from the likes of oxygen and fat, allowing you to maintain efforts for longer without as high of a reliance on glycogen.

Not all long runs are the same and serve the same purpose. Although the main goal being aerobic development there are a variety of different long run types. 

Easy Long Run: Perfect for building an aerobic base with low level stress on the body.

Progressive Long Run: These runs start as a low level of stress but the aim being to progress the effort throughout to finish off harder than you started 

Structured Long Run: These include blocks of sustained efforts often at Marathon/ steady intensities sometimes reaching towards half marathon effort. These might be done to develop race-specific adaptation and mental familiarity with target pace.

Split Long Run: now these are quite prevalent in the ultra distance world, in which you may run for a set period of time one day and then back it up the day later with another longer duration run. These reduce the cumulative fatigue while training for an ultra distance race to still get in the longer sustained effort specific for an upcoming race.

These different types of long runs each come with different rates of metabolic cost on the body to the fuel sources being used for energy and the cost placed on the muscles in terms of fatigue levels post-run. Understanding this cost is crucial, as it influences how you recover, how you structure the rest of your training week, and how consistently you can train across the marathon block.

As long runs extend in duration, the body gradually shifts from relying on carbohydrate stores to increasing its use of fat for fuel. While this is a key adaptation for marathon performance, it also means glycogen depletion becomes a limiting factor, particularly during longer or higher-intensity long runs. As glycogen levels fall, perceived effort rises, running economy declines, and fatigue accumulates more quickly. At the same time, repeated ground contact places significant stress on the muscles and connective tissues of the lower limbs, leading to neuromuscular fatigue, micro-damage, and stiffness—especially in the calves, Achilles, hamstrings, and quads. Long runs that include faster finishes or marathon-pace segments increase this load further, as muscles are required to produce force efficiently while already fatigued.

There is also a systemic cost to consider. Long runs elevate stress hormones such as cortisol, increase overall training stress, and demand longer recovery windows — often 24 to 72 hours depending on duration, intensity, and individual training history. This is why the long run often dictates the structure of the entire training week. If the metabolic and muscular cost is too high, it can compromise key sessions that follow or increase injury risk.

Now after this breakdown of the long run, there are many misunderstandings out there surrounding this activity as well. Many of those misconceptions come from losing sight of why we do them in the first place. 

Running long runs too fast often reflects a misunderstanding of their role in aerobic development. When pace consistently exceeds an easy aerobic effort, the body shifts away from the adaptations we’re trying to target—efficient energy use, improved mitochondrial function, and the ability to sustain effort. Instead of building endurance, the session becomes metabolically costly and harder to recover from.

Treating long runs like a race simulation week after week further increases both metabolic and muscular load. Repeated high-intensity long efforts lead to greater muscle breakdown, elevated fatigue, and a higher risk of injury. Over time, this accumulated fatigue shows up on race day, leaving runners unable to hit their intended pace and turning the marathon into a battle for survival rather than execution.

Under-fuelling long runs is another missed opportunity. These sessions are where runners should practise energy management and learn how the body responds to fueling under fatigue. Likewise, increasing both distance and intensity at the same time often ignores the cumulative stress of long running. Sustainable progress comes from respecting recovery and allowing each long run to build on the last, not compete with it. 

The long run is not about proving fitness — it’s about preparing the system. It develops aerobic capacity, strengthens muscles and connective tissue, improves fuel efficiency, and builds confidence under fatigue. Executed well, it supports every other aspect of marathon training. Executed poorly, it quietly undermines it.  

Written by Sam Goodchild who has an Msc In Sports Health & Exercise Science, who focused primarily on Strength & Conditioning over his masters and placement year. Sam completed his Thesis on the Effect of Strength Conditioning Programs on Running Performance. As well as this is he is the head of Athletics at Nottingham Trent University