I went out for a run Sunday evening to try to do 7-9 miles. My legs felt it today after the long run. My first mile took more than 8 minutes and I knew it was going to be a tough run. My hamstring and piriformis were pretty tight and sore the entire run.
In case you were wondering what the piriformis is, it is a flat muscle, pyramidal in shape, lying almost parallel with the posterior margin of the gluteus medius. It is situated partly within the pelvis against its posterior wall, and partly at the back of the hip-joint. It arises from the front of the sacrum by three fleshy digitations, attached to the portions of bone between the first, second, third, and fourth anterior sacral foramina, and to the grooves leading from the foramina: a few fibers also arise from the margin of the greater sciatic foramen, and from the anterior surface of the sacrotuberous ligament. The muscle passes out of the pelvis through the greater sciatic foramen, the upper part of which it fills, and is inserted by a rounded tendon into the upper border of the greater trochanter behind, but often partly blended with, the common tendon of the obturator internus and gemelli. (courtesy wikipedia)
By the end of the run, I was favoring my leg and I decided to call it quits at 7 miles and about 54 minutes so I didn't end up injuring something. After the run I spent about 30 minutes stretching and foam rolling which seemed to help some.