Towards the end of most of my normal runs and long runs (8-9 miles), something weird starts happening with my quads. They start burning slightly and tightening up so that they become increasingly hard to move. It gets worse after about a mile and doesn't go away unless I stop and stretch them out. I've noticed this before, especially in XC races, but lately it's been happening more often. It never occurs in interval workouts, just distance runs. What's causing it? Should I try foam rolling, stretching more, etc.?