Check on some of the Pigeon stretches or yoga positions and their variations. They are mostly geared for for posterior but I'll tell you it can work the hip flexors too. It will take you a while to get into any of them, so be patient. I've been doing variations of them for almost 6 months and can just now attempt a real pigeon pose, but no where near completing it. And I may never get there. I do the upside down version (thread the needle) and working up to the classic. There is a variation that uses a chair to do standing that I use sometimes.
Supine and seated are shown here.
https://www.peanutbutterrunner.com/four-pigeon-variations-to-stretch-tight-hips/
https://www.healthline.com/health/back-pain/sciatic-stretches#be-safe
If you can't do them on the floor try this. This shows using something to put the leg on.
https://www.menshealth.com/fitness/pigeon-game
Before I got to the classic pigeon I would get on the matt. Put one foot flat in front with knee bent, then push the other leg back with toes back and heel up. Then I would move forward to stretch the quads and in that process I would move side to side some looking for the tight spots. Trying to push the hips down to the floor. The problem with the front leg muscles group is your body is not designed to stretch them. So you really have to work hard to get a good stretch to lengthen those muscles. Mostly by pushing the hips down as much as you can to lengthen those muscles. The cliche runner hamstring/quad stretches are not nearly enough.
Look up kneeling quad stretch. One keeps the leg on the floor. One you can pull the leg up behind you and lean into it. I use my couch to lean on. If you have someone willing to do it to you, look up the Thomas stretch.
I'm no expert on running, but a quick look into it says that the way you are training/running might be causing this. Too many up or down hills, IIRC.
I injured my lower back a couple years ago. The opposite side took most of the work to support the injury and never gave it back. So the good side was over working and the injured side stayed weak. The pigeon stretches have been a game changer for me. I also have a friend that did a couple session of Nero Muscular Reprogramming massage that helped me pinpoint which muscles were turned on (working) and which ones were turned off (not working). That changed some of my focus in stretching.