The longest run you need is shorter than you think.

Somewhere along the way, the long run became a test of nerve. Runners talk about grinding out a 20-miler the way you'd talk about a rite of passage, and plenty quietly believe that if they could just run the full 42 kilometres in training, they'd know they were ready. So they push the long run further and further, certain that longer must mean better. It doesn't. For most marathoners, the longest run you need is shorter than you think, and chasing extra distance past that point costs you more than it gives.

How long does the long run really need to be?

For the vast majority of runners, something around 30 to 32 kilometres is plenty, and roughly 35 is the ceiling. Beyond that, you've stopped buying meaningful fitness and started buying fatigue. The long run's job is to build your aerobic base, teach your body to use fuel efficiently, and get you comfortable on your feet for a long time. Around that two-and-a-bit-hour mark, you've achieved nearly all of that. Every extra kilometre after it adds a little less benefit and a lot more cost, and the curve only gets steeper the further you go.

Why not just run the full distance?

Because the closing 10 kilometres of a marathon, the part you're so anxious about, do not come from having rehearsed them once in training. They come from everything else: the accumulated volume of all your weeks, the experience of running tired, arriving on the day fresh and tapered, fuelling the race properly, and the quiet confidence of knowing you've done the work. Running the full 42 in training gives you almost none of that, and takes a great deal in return. A run that long digs a hole so deep you spend the next two or three weeks climbing out of it, weeks you should be spending training, and it sharply raises your risk of getting hurt right when you can least afford it. You'd be trading away a fortnight of good, consistent work for a single day that mostly just makes you tired. That is a bad deal, and it's one I'd never have a runner take.

How should you build it and use it?

Build the long run gradually across your training block rather than leaping to a big number, and pay more attention to its quality than its raw length. A long run with some of its later kilometres at goal effort, with your race fuelling rehearsed properly, teaches you far more than an aimless slog ten minutes further. Treat it as the place you practise the things that win marathons: pacing with restraint early, taking on carbohydrate at the rate you'll use on race day, holding form when you're tired. What ultimately gets you to the finish strong is not the single longest run in your log. It is the cumulative weight of all the weeks, run consistently and sensibly. Get that right, keep the long run honest rather than heroic, and you'll arrive at the start line both fitter and fresher than the runner who spent the build chasing distance for its own sake.