If you've spotted the words "build week" in your Runna plan and wondered what they mean, you're in the right place.

Build weeks are where your fitness actually grows. They're the engine of your training plan and the reason you'll be running further, faster, and more comfortably by the end of it.

This guide explains what a build week is, why it works, and how to get the most out of one.

A build week is a training week where your overall load increases compared to the previous week. This might mean your runs are longer, the intensity is higher, or some combination of both.

Build weeks are the engine of your training plan. They're the weeks where your body is being pushed to adapt, and where the long-term gains in fitness, endurance, and strength are being laid down.

Your body adapts to training stress. When you run, you create a small amount of physical stress on your muscles, heart, and connective tissue. In the recovery that follows, your body repairs itself and comes back slightly stronger and slightly more capable.

Build weeks deliberately apply a little more stress than the week before, giving your body a new stimulus to adapt to. Over a full training plan, these small, regular increases in load compound into significant improvements in your fitness.

This principle is called progressive overload, and it's one of the most well-established concepts in endurance training. Every effective running plan, from your first 5K to a marathon, is built on it.

Build weeks don't exist in isolation. Your Runna plan is structured in training blocks, typically alternating between build weeks and deload weeks (weeks of intentionally reduced volume).

The pattern looks roughly like this: build, build, deload. Then build again from a slightly higher base.

This structure means you're always moving forward, but never pushing so hard that your body can't keep up. The deload weeks are what allow the build weeks to keep working. Without them, fatigue would accumulate and your progress would stall.

Build weeks should feel like a genuine step up. Not overwhelming, but noticeable. You might feel more tired than usual by the end of the week, and that's okay. That's your body responding to the training stimulus.

A few rough guidelines:

If a build week feels too easy, your plan might be ready to progress and you can adjust your difficulty in the app. If it feels completely unmanageable, you might want to look at your plan length or training preferences.

If you're on a New To Running or Path to parkrun plan, build weeks work a little differently. Because your plan mixes time-based and distance-based sessions and doesn't assign paces, the increase in load shows up as slightly longer sessions rather than changes in pace.

You might not always feel a dramatic difference week to week, but the progression is there. Your plan is building your capacity to run 5K continuously, and every build week is a step closer to that goal.

Don't be surprised if some build weeks on a New To Running or Path to parkrun plan feel very similar to the week before. The increases are intentionally gradual to keep the experience manageable and the injury risk low.

On race and distance plans, build weeks are usually more visible. You'll see your long run distance increase, your weekly mileage climb, and your harder sessions become more demanding.

The build is structured around your goal race date, with mileage and intensity peaking in the final weeks before your taper. Every build week is moving you closer to being ready to run your best on race day.

You can see your build weeks at a glance in your plan's mileage graph, so you always know what's coming and can plan your life around your biggest weeks.

Build weeks are where your fitness is made. They turn the work you've already done into the runner you're becoming, one small increase in load at a time.

Lean into them, recover well between them, and trust the structure your coaches have built. Your future self will thank you on race day!