I Love Running

Hi everyone.
I am hurting after my run a couple of days ago. My quadriceps muscles ached so much I had to stop and walk. I was going around a trail that is about 4 miles long and on the 3rd mile, I gave. What happened??? Usually my ankles hurt, or side pain, or knees, or feet, or shin, or stomach, or lungs. What is sadder than having your quads fail you?