There could be several reasons for this.
The one I see the most often is quad dominance. Sometimes the quads and the lower back become way too strong and they do the job your lower abs and glutes were met to do. The combination of quad/back tends to create a tilt of the pelvis. The anterior tilt of the pelvis sticks the butt out. This can cause a large number of negative outcomes.
First off, it's hard to be fast. If you're not using your glutes to drive your running, you're using something less powerful. In most cases, the glutes are actually shut off, which seems counterintuitive when you see a running picture filled with a prominent butt. Most often in these cases, both the glutes and the psaoas are shut off, and the run is driven by the quads and sometimes the calves. Most people know immediately when they see a runner leaning forward at the waist that they are not "smooth" or "comfortable."
A strong glute runner tends to stand "very tall" and usually has their pelvis in a neutral position.
Secondly, the hamstring is put at risk because it is already stretched because the pelvis is pulling on it. Plus the back usually hurts more as the runner ages because of the poor alignment in the locomotion pattern puts stress on the back that should be on the abs and glutes.
It is not uncommon for shin splits to be a recurring problem for this runner.
You can try to change the tilt of your pelvis just by concentrating, but the tilt was caused by something else in the chain so it's likely to go to a poor default pattern again and again.
Find an Activation specialist who knows locomotion. More and more chiros have learned MAT and understand the importance of glute/psaoas primacy. They can activate your psaoas (which is behind your six pack roughly two inches down and two inches out from your belly button, depending on the person.) They can turn on your glutes as well.
Get your specialist to prescribe the right glute and lower ab exercises for you, but certainly one you'll want to use is straight leg running. Run for 30 or 40 meters without bending your knees or swinging your legs from the sides. Avoid leaning back. Flex your glutes a few times before you do this drill. Do it EVERY time you warm up.
This doesn't go away easily, but when it does, the athletes I've coached become much, much faster and run with much less effort.