Like other tall men, I've never had the best posture. You are most likely slouching due to weakness in your upper back, specifically the thoracic spine. It is not only causing a pelvic tilt, but it is forcing your shoulders to round forward. As I've started working out and lifting, I've noticed a remarkable improvement in my posture. I actually know what it feels like to stop slouching and stand upright. It's not just about having strong glutes but about strengthening the thoracic spine. And the one exercise that has been of most benefit to me in this regard has got to be the pull up.
It not only targets your upper back but also mobilizes the spine and helps strengthen the lats. I have begun performing about 35 daily pull ups, 5 days a week. I am so thankful to God for the results. Right now I believe that good posture is a prerequisite for a quality running performance.
I wish you all the best in your life and your endeavors to be the fittest person you can possibly be.