This article originally appeared Well+Good.

As a runner, you know those days when you feel like you’re running through sand. You’re well-acquainted with runs that leave you gasping for air, and others that make you call it quits early. But what if I told you that by learning to prime the right energy systems (your body has three, by the way), you could build a running performance that made for “no bad days?” Or okay, okay—fewer bad days.

Like shifting gears in your car, certain workouts will activate certain energy systems in your body. This happens on autopilot because your body’s just that cool, but once you get to know what activities prompt your muscles to tap into different power sources, you can get smart about programming your workouts. To make it easier for you, I have provided a go-to guide on how to structure your weekly workouts, what types of workouts to do, and when to do them based on your trifecta of energy systems.