What is the best way to train for a marathon?

Running ability depends on three main factors, each partly genetic but all amenable to training. The first is the lactate threshold: the point at which lactate builds up in the blood faster than it is cleared away. Lactate is a byproduct of metabolism and the threshold marks the highest intensity at which the body can meet energy demands aerobically before shifting to less efficient pathways. The second factor is VO₂ max, the maximum rate at which the body can deliver and use oxygen during exercise. Both of these can improve with training.

The third factor is how much energy a person expends to cover a given distance in a given time. This depends on both body shape and technique, and tends to change more slowly.

There have been no high-quality randomised controlled trials comparing the effectiveness of different marathon-training plans, but analysis of large datasets of active runners can provide insights. One study, published in 2024 by researchers based across Britain, Ireland and New Zealand, analysed 16 weeks of training data from roughly 120,000 marathon runners who logged their runs on Strava, an exercise-tracking app.

Their primary finding was that distance matters during training. The fastest marathoners—those finishing in two to two-and-a-half hours—covered roughly three times as many kilometres per week as the slowest. The number of long runs completed and the number of days spent training were also strongly correlated with faster times. For example, runners whose training involved ten long runs (those over 20km) were, on average, nearly 50 minutes faster, in the final race, than those who completed none.

Crucially, much of this training was done at low intensity. Almost all the extra distance logged by the eventual faster runners was completed at an “easy" pace, slower than marathon speed and below the lactate threshold. Lower-intensity training seems to improve aspects of running physiology with lower risk of injury and less need for long recovery breaks.

Elite athletes typically spend around 80% of their time running slowly but for casual runners this can seem counter-intuitive. Indeed, Cailbhe Doherty of University College Dublin says that most casual runners are “working too hard" during their training.

Even as a race draws near, there are still measures that can be taken to improve performance. Tapering training becomes critical—cutting mileage three weeks before the event, rather than just one, can improve finishing times by up to 2.6%. This rest allows muscles to replenish their glycogen stores (chains of glucose molecules).

Carbohydrate loading in the final 48 hours is vital for the same reason. Dr Doherty recommends consuming eight to ten grams of carbohydrate per kilogram of body weight—a surprisingly difficult target for most people to hit.

In summary, run at a comfortable pace as often as possible for as long as possible. It may not be time-efficient, but it seems that the best way to race fast is to train slow.