hip mobility

Do any of runners have any suggestions for alleviating sore/tight hip flexors when running? It seems to come on after about 10-12km on the flat and a bit sooner if there's lots of uphill. I run 6.6km once a week without any issues but in the last month have done a 14km and yesterday a 20km...

Hello. Do I need a specific hip rotation exercises? Or just stretching and strengthening the hip flexors, glutes, adductors/abductors and hamstrings should be enough? I have problems with my internal hip rotation. In the book "Relax Into Stretch" there was no information about doing exercises...