Sit ups question concerning leg strength.

I have been working on my sit ups for a while now. I can't seem to get p the point where I can do fifty in a minute. I would like to get to where I can do 60 or seventy in a minute. My muscle in my hips just give out after about 50. What can I do to improve my hip strength to where I can achieve my goal? What would be some good exercises. Would squats be good? I have been doing situps for about two months now, how long should it take to see results?