As for the number of sets and reps, that depends on what your goal is, and what part of periodization you are in. If you are aiming at increasing strength and speed in running i suggest you do 1-4 reps at above 85% for all exercises. I am a sprinter and this is what I do. It will increase your speed potential, so that you exert less force when you run at X m/s than before. Doing high force/low reps increases neural output and muscular force production, which will make any subsequent training easier and more efficient. For example, if you get 10% faster, you will run 10% faster (or longer in the same time period) in your distance training, and as such increase intensity/volume and fitness. There might be some merit in doing high reps, >20-30, in long distance running. However, the main aspect of increasing performance in long distance running should be running itself. With strength training being an add-on.
As for the type of exercise, the prime movers in running are the hip extensors (glutes and hamstrings). Adductors also play a role in extending the hip, but not as much for long distance as compared to sprinting. Abductors are mainly stabilizers in running. It makes sense to strengthen them in case your hip tilts when you run; they only need to be adequately strong; if you don't tilt your hip toward the end of the runs you have no need for extra strength there. Hip flexors are secondary movers in running; very important for sprinting, but not so much in long distance running.
If you are aiming at maximizing performance in your long distance running with minimal time investment, then focus on hip extensors. If you have time, then you can add flexors, adductors and abductors to your regimen, in that order. For more efficient training, look up "periodization". I would recommend Periodization for Sports by Tudor O. Bompa.