I'm glad you shared your experience about situps. That's a VERY common result of doing resistance training without increasing the resistance. Imagine instead of increasing the number of repetitions infinitely that you simply held a 5-pound plate across your chest for a week or 2, then gradually added more and more weight, keeping the number of repetitions the same.
I can see what likely happened in your case. Your abs were sufficiently developed such that they completely took your hip flexor muscles out of the picture when doing situps. Adding reps didn't help because the muscles didn't get fatigued.
If you had increased the resistance over time, it is likely that you would have needed to use your hip flexors more, and wouldn't have to have added a 2nd exercise. Furthermore, you would have shortened the overall ab exercise time, and not gotten so bored of situps!
Personally, I do about 5-10 minutes of ab exercises twice a week. I do 3 sets of 10 reps of incline situps, with 30 pounds on my chest. I also sometimes do crunches with 55 pounds on my chest, and occasionally hanging leg raises, with a 30-pound dumbell held between my ankles.
Along with heavy squats and deadlifts, my ab region is adequately stimulated.
A general point: if your goals are to increase strength/muscle mass, and you are able to easily complete 12 controlled, slowish repetitions of a particular exercise, then you should add resistance the next time you do that exercise. Doing 20, 50, or 1000 repetitions will not help your strength or muscle mass; it will help muscular endurance. Note, it will NOT help "tone" muscles, which is (to state this again) simply the lack of fat surrounding your muscles, enabling them to be seen. Getting rid of fat is part diet, part burning calories through cardio-vascular exercise, and part building muscle to increase your body's fat-burning efficiency.
One final point: to anyone who avoids resistance training because they are worried about developing freakishly large muscles, don't worry! You WILL NOT develop such muscles unless you are a genetic freak. Resistance training is a marvellously efficient way to stay in shape, to slow the aging process, and to keep bones and muscles healthy. Combined with heart-friendly cardio exercise and a sensible diet, and you'll look and feel better than ever. Cardio by itself is good, but doesn't have any of the muscle/bone/aging benefits of resistance training.
Sorry for the ramble!