Whenever I do a long run (10-15 miles), it seems regardless of the pace, after maybe the 8th mile or so it feels like my hips (along with my legs) always feel very fatigued and it feels harder to maintain good form. I'm not necessarily breathing hard at all and good maintain a conversation, but my lower body always feels very tired at the end of any long run, even if I went 8:00+ pace, because of just time on my feet.