Agreed - i get my guys (and girls) to learn to Turn their abs on during tempo 1st to learn how to do this. When they activate their lower abs, you can see their hips rotate, their feet land better, their posture improves etc and they FEEL like they are running easier. Its like a light you turn on and off by turning the abs on and off.
Once one has mastered it at tempo speed, it becomes easier to attempt it at high speed. (is it worth doing high speed if you cannot even keep your hips aligned at tempo speed?)
Also good practice to turn abs on and focus on hip alignment during Running A’s. A lot of people just ‘do it’ and as you say, tire out their hip flexors and try to keep going under Poor form.
Lying Leg Raises, focusing on the lower back being forced onto the ground at all times, not allowing it to arch up at all is a great exercise.