Here’s an article on the importance of engaging your glutes when you run.

Doing dynamic stretches or warm-ups before running has been critical for me. When I first started running, I had recurrent posterior tibial tendonitis, and later a bit of plantar fasciitis – both were linked to a failure to engage the butt. You should definitely do drills that trigger the glutes before you run. This gets them engaged and firing once you’re going – otherwise, as the article says, all the smaller muscles are doing all the work, and that’s a sure path to strain and injury.

One-legged squats are a great way to strengthen your bum – if you’ve never done them, you’ll wobble all over the place and feel stupid, but stick with it. You’ll strengthen up quickly, and pretty soon you’ll go from one rep at 150 degrees and falling on your ass to 10 reps at 90 degrees looking like a still-shot of one of those Cossack dancers.