I strained my hip flexor last year when training for a half marathon. The pain was at the top of my leg, right at the joint and toward the inner part of my thigh. It was painful to lift my leg when going up stairs, or if I was lying flat on the ground on my back, it would be very painful to try to lift my leg with the hip flexor strain.
I rested for about a week, stretched quite a bit, and from then on I closely monitored the pain, and would always ice and stretch the area after my runs. I could still feel it flare up a bit, but it gradually improved. I think my problem was I had completely neglected those muscles when doing strength training. So now I incorporate that into my workout, and once the strain had fully healed, I did more hill workouts, because it forced me to lift my legs higher and work my hip flexors more.