Trail running is a sport where performance depends as much on physical endurance as on the ability to cope with elevation changes and technical terrain. Building a solid endurance base is therefore essential: it is the foundation on which you can then develop speed, climbing power and mental strength. Without this foundation, you risk injury or early fatigue and will not get the full benefit of your specific training. This article details the principles and methods for building your endurance in a sustainable and safe way.

What is an endurance base?

Your endurance base corresponds to your body's aerobic capacity. It is built up mainly by practising basic endurance: running at a comfortable pace where you can still hold a conversation. This intensity represents about 60 to 70% of your maximum aerobic speed (MAS) or maximum heart rate. Slow jogging improves the efficiency of the heart, develops the capillary network, promotes better oxygenation of the muscles and optimises recovery.

To structure your week, the 80/20 rule is a useful guideline: approximately 80% of your training should be low intensity and 20% moderate or high intensity. This distribution can be adjusted according to your level and how you feel.

Why is this foundation essential in trail running?

In trail running, rough terrain, long ascents and descents, and the duration of the events place heavy demands on the cardiovascular system. A solid foundation allows you to absorb specific training sessions and avoid overtraining. It also limits the risk of injuries caused by overexertion thanks to a gradual increase in volume.

This phase develops cardiovascular endurance by focusing on light to moderate effort. It is also an ideal time to strengthen the body and work on technique gently, especially on descents, without excessive fatigue. Finally, building your base builds mental resilience, a crucial asset over long distances.

Principles for building your base

Gradual progression and volume

The key is to increase your running volume gradually. At the beginning of a cycle or after a break, start with less than 50% of your usual mileage and increase by 5 to 10% each week. This limited progression protects against injury and allows time for your tissues to adapt.

During the general preparation phase, focus on basic endurance runs, general physical preparation (GPP) and short hill sessions for a little speed. The aim is to build a solid foundation and muscle base that will enable you to cope with future training sessions.

Intensity and training zones

Stay mainly in the basic endurance zone (60–70% VMA or maximum heart rate). Once you feel more comfortable, add a weekly active endurance or tempo session at around 80% of your maximum heart rate. This zone corresponds to the aerobic threshold and improves your ability to sustain moderate effort.

Save high-intensity intervals (VMA, short intervals) for the end of the period or to vary your training, while sticking to the principle of around 80% easy work and 20% more intense work.

Long runs and back-to-back runs

The long run is the cornerstone of endurance building. Plan one long run per week, gradually increasing its duration. On trail terrain, focus on time spent running rather than distance covered, as the pace is often slower.

Avoid increasing your mileage by more than 10% per week and insert a lighter week every 3 or 4 weeks to allow your body to recover. When preparing for very long distances (ultra), consider back-to-back runs: two consecutive long runs (e.g. two 15 km runs) to simulate the fatigue of a long race without running an ultra in one go.

Hill work and technical terrain

To build power and prepare your body for elevation changes, include hill sessions in your training. Hill repeats (6 to 10 intervals of 30 to 90 seconds on a steep slope with a jog down) develop muscle strength and endurance.

Prolonged endurance climbs and controlled descent sessions strengthen the quadriceps and improve your descent technique. At the same time, spend time on technical trails to improve your balance, coordination and stabiliser muscle engagement. Start with moderately uneven terrain and gradually increase the technicality over the weeks.

Cross-training and muscle strengthening

Cross-training is a valuable ally in building a foundation without overloading your joints. Cycling, swimming, cross-country skiing, and hiking improve cardiovascular endurance while reducing impact. These disciplines also allow you to vary your activities and avoid monotony.

Don't forget general physical preparation: exercises to strengthen your core, glutes, hamstrings and calves (squats, lunges, core training, step-ups, etc.) strengthen the muscle groups used when climbing and prevent injuries. During the base period, one PPG session per week is sufficient.

Moderate speed sessions (tempo/intervals)

Even though most of the work is done in endurance mode, it is useful to incorporate moderate tempo or interval sessions to maintain your ‘engine’. Example: 4 to 6 times 3 minutes at a sustained pace (zone 3-4) with 2 minutes of recovery jogging.

These sessions improve VO2max and tolerance to sustained effort, while remaining compatible with a basic phase. Be sure to maintain controlled execution and recover well afterwards.

Nutrition, hydration and recovery

Building your base also requires proper nutrition and hydration. During long runs, practise consuming carbohydrates regularly (gels, dried fruit, bars) and drinking water or electrolyte drinks. Learn what suits your stomach to avoid problems during the race.

Allow yourself at least one full day of rest per week and schedule light recovery weeks every 3 or 4 weeks. Gentle activities (yoga, stretching, self-massage) promote regeneration and reduce the risk of injury.

Mental and technical

The base phase is ideal for developing mental resilience. Long endurance sessions and hours spent strengthening build patience and confidence. Work on your downhill and support technique by incorporating specific exercises and varying the terrain. Good technique reduces fatigue and improves running economy.

Example of a basic training week

This structure is flexible. Beginners can reduce the volume and skip the tempo session, while experienced runners can increase the duration of long runs or add a second threshold workout (e.g. fartlek). Always listen to your body and adjust the programme accordingly.

Tailor your training to your circumstances

Every trail runner has different circumstances and goals. If you have a busy schedule, reduce the frequency of your sessions while maintaining regularity: two to three basic endurance runs and a long weekend are enough to maintain your base fitness.

Athletes preparing for ultras can schedule back-to-back runs to simulate cumulative fatigue. Beginners should extend the base phase (up to 8 to 12 weeks) and include more cross-training to limit impact. Experienced runners can shorten the phase (4 to 6 weeks) while adding more frequent threshold sessions.

Conclusion

Building a solid endurance base is the key to progressing in trail running. By structuring your training around gradual progression, a predominantly low-intensity volume and regular long runs, you prepare your body to absorb future specific loads.

Hill sessions, cross-training, strength training and technical work enrich this phase by strengthening muscles and improving coordination. Patience, recovery and consistency are your best allies. With a solid foundation, you can then safely increase the intensity and get closer to your goals, whether it's your first 20 km in the mountains or a 100 km ultra.