So yesterday I ran my first interval session of Daniels' 10k plan (4 x 1200m w/ 3 min jog) and it destroyed me. Partly it was overestimating my fitness and part of it was from getting like two hours of sleep the night before, so I went in super fatigued. I should have known better and I get where I screwed up on that. But during the start of the third one, I felt my glutes were fatigued and it got worse over the course of the third and the final one. Now, this has never happened to me before. Is this something that happens when you're training at too hard of a pace? Or is this a muscular issue, ie I need to do some squats?