For most people, running fatigue is a state of feeling overly tired or constantly fighting injuries. It usually builds up over several months and somewhat differs from normal fatigue caused by consistent training. With normal running fatigue, your legs may feel tired some days. This is what marathon runners call “cumulative fatigue.” This type of fatigue is a good thing. It trains your body to work through fatigue and tired legs, which can help you finish out marathon races without feeling absolutely exhausted at the end. When your legs are fatigued, many runners recommend running at a slower pace. Running through sickness or pain is not ideal, but tired legs can still do the work at a slower pace. On the other hand, excessive running fatigue is not a good thing, and running through it can cause more severe issues down the line. For example, if you can never hit your intended paces and your legs always feel tired, you’re probably too fatigued, and it’s time to take a break. The key is finding a balance between cumulative fatigue and excessive fatigue to improve your aerobic fitness without overdoing it.