If you’ve never been taught how to fuel for long training runs as you prepare for a half-marathon or marathon, it’s definitely a good fit.

You’ve been faithfully following the training program: putting in the miles and doing the strength circuits. You’re going to bed before 10 pm, you’ve eliminated your nightly glass of wine, and you’re even foam rolling as you watch Queer Eye on Netflix.

You’re doing everything right, so why do you lose steam during your long runs—and sometimes even your shorter ones? Chances are, it’s your nutrition. You’re not properly fueling before and after, and most importantly, during your runs. And when you don’t eat for running performance and endurance, your GI tract might revolt. Your legs might feel heavy. You might get a killer headache or you might feel nauseous. Or the running-sucks-or-maybe-I-just-suck track might play on repeat in your head. Or you simply might incrementally peter out, like a car whose gas gauge inches towards E.