Tightness, tenderness and slight strains in the upper hamstrings and butt are going to be the bane of most ambitious runners at some point, especially ones who sit a lot in their non-running time. (Yes, that again! It really is significant.) During a flare-up, avoid steep uphills, and ease into all your runs. Make sure you’re fully warmed up before hard workouts. If the sensation is of tenderness rather than tightness, don’t stretch the area while it’s in an acute phase, as that will just further irritate the already strained muscle fibers. Ice the area after all runs.
Once the acute tenderness is gone, be relentless about stretching your hip rotators and hamstrings, but never to the point that you reproduce the pain. Strengthening your hamstrings with single-leg curls will help to restore the proper balance of strength between your quads and hamstrings. And do what you can in your non-running time to sit with good posture and to get out of sitting postures at least once an hour.