During my last long run I felt soreness in the muscle that runs up to the groin. Not terrible, but it hurt. Walking for the past couple of days I felt no pain at all. Today I did a 3 mile easy run on the treadmill, and at the end that muscle was sore again. I haven't had any problems with it before. I walked for a while and it didn't hurt at all, but when I attempted to speed up again, I really felt it, so I slowed to a walk for another half mile or so. What am I doing wrong?
Maybe consider some mild stretching exercises (inside leg raises) to strengthen the adductor muscle, and think about using elastic bandage/tape and cutting back your running to aid in recovery. The only other thought is to double-check for some sort of foot imbalance that may be the primary cause.