I know of two good hip flexor/extensor exercises. They both look weird, but in my experience, they help. One is to sit on the ground in a hurdle-type position. Then lift your front leg off the ground using only your hip muscles. Don’t slouch–you’ll use your stomach muscles to cheat. Try to do ten in a row without resting on the ground, then switch legs. The other is to sit on the ground with your legs out in front of you like you’re going to do a pike stretch. Then use one leg at a time to kind of scoot along on your butt. Make sure you don’t slouch and keep your legs as straight as possible. Hope that helps.