Intensity can drive quick improvements in trail running performance—but only for a short time. Hard sessions create a strong training signal, yet they also generate fatigue faster than they build fitness. At first, fitness gains are visible; over time, accumulated fatigue masks those gains, leaving runners feeling heavy, flat, and slower despite training harder.

The mistake many runners make is responding to this fatigue by adding even more intensity or letting easy runs drift too hard. Instead, sustainable progress comes from using intensity sparingly, building a strong aerobic base, and allowing recovery to keep pace with training stress.

Key message: intensity should support training, not dominate it. Rule of thumb: If intensity is always the solution, it eventually becomes the problem.

Most ultra runners don’t lose races on the climbs—they lose them on the descents.

In this episode of The Trail Running Briefing, we break down why downhill running causes so much damage despite feeling easy at the time. You’ll learn how eccentric muscle loading silently destroys the quads, why this fatigue is delayed and deceptive, and why cardiovascular fitness alone won’t protect you late in an ultra.

We also cover the most common mistakes runners make in training—avoiding downhills, underestimating their impact, and treating them as free speed—and what actually works instead. From smarter downhill exposure to eccentric strength work and technique adjustments, this episode gives you a simple mental model to understand why so many races fall apart after the halfway point.

Key takeaway: If you don’t train your quads for the downhills, the race will.

Understand your training. Run better.