The best running tip I've ever had is "run tall". Most people tend to slouch forward at the hip which prevents some muscle groups from engaging. Made a world of difference for me.
My version of "run tall" is "pretend you're riding a pony". Your hips and legs are the pony, and your torso is riding it, and it works for me as a cue to remember that my torso should stay tall and upright.
Also when I get really tired, it helps me dissociate myself from the feeling in my legs because instead of thinking "ow my legs hurt", I can think "thank you, pony, for being so strong and working so hard for me!"
Yeah, those two words imply a bunch of posture/style fixes. "Run tall" and "steady breathing" are the two simple reminders that I repeat to myself if I'm getting tired.
