Running (Hip flexor question)

When training for my HIM and doing my long runs, the route I run has about 50% hills and 50% flats.  Since my HIM course is all flat, I found a completely flat route to run.  I have run this new route two times now.  1st - 11 miles.  2nd - 5.2 miles.  Both times, my hip flexors were real tight near the end of my run and several days after.  The thing that confuses me is that the first 2.5 miles of my old route is flat, followed by 2.5 of hills, then flat again and I have never had any issues.  My race is in one week.  Since this has happened, I have really focused on stretching my hf as well as every other muscle in my lower body.  I am a little concerned about this.  I am hoping that after the ride, they will be stretched enough to not cause too many issues.  Any thoughts on this?