Hip flexor pain on long runs

Hi! I’m training for my first marathon in two weeks, and for the past few long runs I’ve been hit with such painful hip flexors. This happens at about 14km in and then I barely make it to 26km because they’re so painful.

My usual running pace of 5:55 per km got taken down to 7 min per km on my 22km long run yesterday, so I’m feeling pretty deflated.

I stretch them almost every day so I’m unsure if that’s the problem? I’ve maintained strength training at least once a week the entire block. I’ve read it could be core strength, so I’m going to work on that solidly until the race and hope for the best.

Any advice or insights? They’re all welcome!