The long run

The useful rule is proportional, not absolute. Your long run should be roughly 25–30% of your recent weekly volume — where "recent" means the average of the last three or four weeks you genuinely completed, not the week you had planned.

Two adjustments. If you run only three days a week the share creeps up — 30–35% is normal and hard to avoid, because there are fewer days to spread the volume across. And if you are marathon training, the long run legitimately exceeds 30% during peak weeks; that is a deliberate, temporary distortion for race specificity, not a general principle.

The failure this rule prevents is the most common one in amateur running: a 32 km Sunday hanging off 45 km weeks. That long run is 70% of the week, it takes four days to recover from, it degrades every other session, and it predicts injury better than it predicts a good marathon.

Cap it in minutes, not kilometres

A 30 km long run is a two-hour job for a fast runner and a three-and-a-half-hour job for someone finishing a marathon in 5:30 — the same number on paper, a completely different stress. Most non-marathon training is well served by 60–105 minutes; marathon training caps somewhere between 2:30 and 3:00 whatever distance that produces. If a plan's distance would take you more than three hours, run the time cap and let the distance be what it is. Past about three hours the injury and recovery cost rises faster than the fitness returned.

This is the situation almost every runner is actually in, and the one every distance ladder ignores. Week 9 of a printed plan asks for 26 km because it assumes you completed weeks 1 through 8. If you did not, that number is a fiction, and running it anyway is how people get hurt in the last month before a race.

Whatever the case, the number to re-derive from is your recent completed weekly volume. Almost every bad decision here comes from anchoring to a plan you did not execute, or to the fittest you have ever been, instead of to the last three weeks you genuinely ran.

Easy, conversational effort throughout — full sentences, not gasped fragments. For many runners that is 60–90 seconds per kilometre slower than the pace they think of as normal, and it will feel almost embarrassing for the first few weeks.

Running it too fast is costly in a specific way. A long run at moderate effort is hard enough to require real recovery but not hard enough to be a quality session — so it takes the sharpness out of the week's threshold work and gives nothing back. You end up with a week containing two mediocre hard days and no easy ones.

Hold effort constant, not pace: heat, hills and fatigue all mean the same effort produces a slower pace, and chasing the number on those days is how a long run turns into a hard one. A little drift late in the run is normal — you are tired. Fighting to keep the pace flat at the end is not the same as finishing strong.

Three things, and they justify different treatments of it.

• Aerobic development. Sustained low-intensity running drives capillary growth in working muscle, mitochondrial density, plasma volume, and a heart that moves more blood per beat.
• Durability. Time on your feet conditions tendon, bone and connective tissue to tolerate repeated loading. This adapts far more slowly than your cardiovascular system does — which is exactly why distance has to be built patiently even when your lungs are fine with it.
• Rehearsal. Beyond about 90 minutes, it is where you practise fuelling, pacing, kit and the business of continuing when it stops being fun. Not fitness — and the reason marathon runners keep long runs even in weeks the physiology does not demand one.

Note what is missing: it is not primarily a speed session, and it is not a weekly test of how far you can go. Treating it as either is the root of most long-run mistakes.

An all-easy long run is correct for most runners most of the time. Quality inside it is a race-specific tool, and only makes sense once the easy version is comfortable and habitual.

• Strides at the end. 4–6 × 20 seconds relaxed and quick. Cheap, low risk, a good counter to the shuffling gait long slow running encourages.
• Fast finish. The last 10–15 minutes at a steady, controlled effort — harder than easy, well short of threshold. The best first step into quality.
• Race-pace segments. For marathon runners in the last eight weeks: blocks such as 3 × 15 minutes at goal marathon pace inside an otherwise easy run. The most specific session in marathon training, and the most commonly over-cooked.

Three rules keep this from backfiring. A long run with quality in it is a hard day — count it as your hard session, or keep the week's other one far away from it. Do not add quality in the same week you extend the distance. And never turn the whole thing into a tempo run: the length is the stimulus, and running it fast means making it short.

Below about 75 minutes, water is generally enough for most people. Beyond roughly 90 minutes the long run becomes the place you practise fuelling — and the practice matters as much as the fuel, because the gut adapts to taking on carbohydrate while running and race day is a bad time to discover it has not.

The commonly cited guidance is 30–60 g of carbohydrate per hour for efforts over 90 minutes, taken at regular intervals rather than all at once, with higher intakes used by experienced endurance athletes who have trained their gut for it. Start at the bottom of that range and build. Use the product you plan to use on race day, at the intervals you plan to use it, in the same pockets or belt — and the same for shoes, socks, chafing, hat, and what happens in rain.

A runner averaging 40 km across four days, targeting a half marathon in twelve weeks. Long run starts at 12 km, easy throughout.

• Weeks 1–2: 12 km. Weeks 3–4: 14 km.
• Week 5: cut back to 10 km, hold the weekly volume.
• Weeks 6–7: 16 km. Week 8: 18 km, last 12 minutes at a steady effort.
• Week 9: cut back to 12 km.
• Week 10: 19 km, the longest of the block. Week 11: 14 km.
• Week 12: 10 km easy, race weekend.

Now suppose weeks 6 and 7 disappear to a chest infection. The plan says 18 km with a steady finish. What you should actually run is around 11 km, all easy — roughly 75% of the 14 km you last completed — then rebuild, accepting that the peak long run this cycle will be nearer 16 km than 19 km. That race will be fine. The one you run on a body that skipped the rebuild will not be.