Getting into trail running, from the road or from scratch

What changes when you leave the road

2.5 cm. That was the height of the biggest bumps on an ‘uneven’ treadmill used by researchers at the University of Michigan. It was enough to make the twelve runners tested burn 5% more energy than at the same pace on a smooth belt, and the width and length of their steps became about a quarter more irregular [1]. Now picture the roots, stones, mud and steps of a real path.

Terrain is the first change, and the one road runners most often underestimate. The others come down to relief: climbing costs more and more energy as the slope steepens, and descending makes your muscles work as brakes [2]. Your pace per kilometre, so precious on tarmac, loses much of its meaning. And if you are starting from scratch, with no running past at all, I would almost call it an advantage: you have no road habits to unlearn.

Climbs: walking is not cheating

On the road, walking feels like an admission of defeat. On a steep trail, it is often the most economical choice. At the University of Lausanne, researchers had seventeen endurance runners walk and run on slopes of varying steepness, always at the same rate of climb: 800 m of ascent per hour. Beyond an average gradient of 7.2°, roughly 13%, they spontaneously preferred to walk, and that was about the point at which walking became cheaper than running [3]. Your body does the maths for you: let it, without embarrassment.

Descents: less puffing, more work for the thighs

A descent does not leave you gasping, and that is precisely the trap. Going up, your muscles produce work to lift the body; going down, they mostly absorb it by braking, as a 2017 review of the research puts it [2]. The energy cost of running falls on gentle downhill slopes to a minimum at around -20%, then rises again as the slope steepens [2]. You puff less, but your thighs take the strain. Our training plan for a first trail race explains how to get your legs used to descents, with the research behind it.

Thinking in hours rather than kilometres

On the road, a 10 km is a 10 km. On trails, the same distance can hide a gentle jog or half a day in the mountains. The Fell Runners Association warns newcomers to fell runningHill and mountain running in Britain, often off paths and on unmarked courses, where runners navigate with map and compass. that a 10 km fell race is likely to be a much tougher undertaking than a 10 km trail race, and that it will take longer, ‘often much longer’, than a race of the same length on the flat [4].

Even at the top level, the clock reads in hours. For the short trail race at its world championships, 35 to 45 km with 2,000 to 3,000 m of ascentThe sum of all the climbs on a route, in metres., World Athletics aims for a men’s winning time of between 3 hours 45 and 4 hours 15 [5]. To compare two routes, the trail world therefore adds distance and climb together: this is the km-effort, 1 km for each kilometre covered plus 1 km for every 100 m of ascent [5, 6]. A 20 km route with 800 m of ascent comes to 28 km-effort.

To estimate how long a run will take, your own figures are the most reliable guide: note the time, distance and ascent every time, and you will soon know your own rhythm, much as walkers do with our guide to estimating walking time.

The minimum kit: what race rules really ask for

The good news: for a first run on a woodland track, the list is short. Shoes suited to the ground, first of all: drop, cushioning and lugs are explained in our guide to trail running shoes. Quick-drying clothes, layered when it is cool according to the three-layer system. Water, and a charged phone. The rest depends on how long, how high, the season and how remote you will be, and that is where race rules become handy checklists, even for training.

Their logic is the same everywhere. In France, the rules of the French athletics federation (FFA) require the organiser of a trail race to impose the kit that allows runners, throughout the event, to avoid getting into distress (a light, clothing, water, food) and, in the event of an accident, to raise the alarm (a whistle, a phone) and wait safely for help (a survival blanket, extra clothing) [6]. World Athletics Rule 57 says much the same: depending on the expected conditions, organisers may recommend or impose minimum clothing, a windproof jacket, a head torch, water or food [5]. The exact list, and the amount of water, are set by the organiser, who knows the terrain and the season [6].

The detail that changes everything is a single sentence in the UTMB rules: it is important ‘not to choose the lightest possible clothing’ [7]. A jacket that ticks the box but lets the rain through is no use on the day it pours. The Fell Runners Association puts it its own way: its mandatory kit is ‘the bare minimum’, to be topped up according to conditions and experience [4]. One more, more recent point for races in France: since the FFA’s 2027 rules, trail races must replace plastic cups and bottles by refilling runners’ own containers [6]. Bring your soft flask or a cup.

Your first runs: building up without rushing

If you come from the road, the temptation is to keep your numbers: same mileage, same pace, just somewhere else. If you come from nowhere, it is the opposite: you have no idea where to start. Either way, I would begin on broad, easy tracks close to home, before seeking out narrow paths and the mountains.

Then how much should you add from one week to the next? The honest answer is that nobody knows to the nearest per cent. The famous 10% weekly rule was put to the test in the Netherlands, without result [9]; a Danish study suggests being wary of big jumps instead [10].

Those distance-related injuries include iliotibial band syndrome and patellofemoral pain, and the Danish authors themselves describe their result as exploratory [10]. My takeaway: build up in small steps, with no sudden jump from one week to the next, and count the climbing as a load in its own right, since going up takes more energy and going down works the muscles differently [2]. For a week-by-week programme, our plan for a first trail race takes over.

Technical ground, for its part, is tamed by slowing down. In an American study published in 2026, 27% of the 227 runners followed across nine trail races of 20 to 50 km reported a fall, and 23% a rolled ankle [11]. More surprisingly, these incidents were more common in the faster races with less climbing, and on narrow singletrack; pace and perceived effort predicted them better than anything else [11]. Easing off on tricky sections, especially as tiredness sets in, remains the first line of defence.

Alone on the trail: weather, water and route

In a race, an organiser marks the course, staffs the aid stations, logs runners at checkpoints and sweeps the course behind the last runner. Even so, the French federation’s rules remind runners that they are ‘the first link in the safety chain’ [6]. Out training, alone on a trail, you are the only link: a few simple habits, taken from official advice, make all the difference.

Common mistakes when starting out

• Setting off at road pace. Better a pace at which you can still talk on the first climb: in the study on falls, speed and perceived effort were the best indicators of risk [11].
• Trying to run every climb. Beyond a certain gradient, walking costs less energy than running [3]: a brisk walk is a technique, not a surrender.
• Forgetting the descents. They do not leave you breathless, but they load the muscles through braking [2]: train for them as you would for the climbs.
• Treating the kit list as a ceiling. It is a minimum, to be chosen for real protection rather than the lowest weight [4, 7].
• Cutting the corners of zigzag paths. The French federation’s 2027 rules require organisers to ban leaving the marked paths, with disqualification among the possible consequences [6].
• Relying entirely on your phone. There is not a signal everywhere [8], and a flat battery calls nobody.

Your first race: formats, categories and entry

Choosing a format

There is no official distance in trail running: neither a set length nor a required amount of climbing, as World Athletics and the French federation both point out [5, 6]. A race must, however, take place mostly off roads, with no more than 25% on paved surfaces, and be marked so that no navigation skills are needed [5, 6]. To make sense of it all, the French federation sorts trail races into seven km-effort categories, from XXS, up to 24, to XXL, from 210; these are the bands used by the International Trail Running Association (ITRA) and adopted by World Athletics [5, 6].

Enlarge diagram

For a first race, I would go for an XXS: short enough to finish with a smile, long enough to discover climbs, descents and an aid station. Km-effort measures effort, not technical difficulty: a runnable 15 km through woodland and a rocky 15 km in the mountains can land in the same category without having anything in common. The profile and the course description fill in what the number leaves out.

Reading the race rules

Before you enter, read the race rules from start to finish. World Athletics requires organisers to publish at least the programme and any cut-off times, the distance, the ascent and descent and the main difficulties, the location of aid stations, a map and a profile, the permitted, recommended or mandatory kit, and the safety rules [5]. Many trail races are run on a semi-self-sufficient basis: you must then carry enough water and food to get from one aid station to the next [6]. In France, cut-off times are compulsory above 40 km-effort, and a runner who drops out must tell the organisers as soon as possible [6].

Enlarge photo

Entering a race in France: licence or Pass prévention santé

For a trail race in France, adults no longer need a medical certificate: the French athletics federation has replaced it with the Pass prévention santé (PPS), in line with a law of 2 March 2022 [15]. Its running events rules, applicable since 14 October 2026 to all organisers, affiliated or not, require adults to show one of two documents [6]:

• a French athletics federation licence (Athlé Compétition, Athlé Entreprise or Athlé Running) valid on race day; the Santé and Encadrement licences do not count;
• or the PPS, issued no more than a year before the race: an online health awareness course costing €5 for a year, which includes third-party liability cover [15].

Foreign licences are not accepted, and nor are licences from other French federations, unless they have an express partnership with the FFA; the current rules list none [6]. Minors show a French athletics licence or, depending on the organiser, a health questionnaire completed with their parents, free of charge and repeated for each race; a single ‘yes’ answer then means a medical certificate less than six months old [6, 15]. There are limits too: 20 km-effort at most for 16- and 17-year-olds, and any trail race with more than 950 m of ascent is open only to those aged 18 and over [6]. These rules apply to races in France; elsewhere, including in the UK, check each race’s own entry conditions.

Which leaves the best part: finding your first trail. It often begins where the tarmac ends, a few minutes from your door, with a climb you will walk without blushing and a descent that, the first time, is as much a giggle as a wobble.

Worth watching

The Run Experience shows what to change on steep trails, both on the way up and on the way down: the ground that most sets trail running apart from the road.

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Nolwenn Bastide

Trail running, snow, water, nature and destinations

Nolwenn covers trail running, snow, water, nature and destinations for Outfarer. Curious about everything, from meteor showers to night-train timetables, she has a knack for unearthing the one detail that changes everything, and putting it in a single sentence.

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