abs are like any other muscle, they shouldn't be worked every day, also rather than doing heaps of sit up make the situps harder or add in some variety with different ab exercises.
you are not going to be able to spot reduce the fat over your lower abs to see definition there you need to work towards a very low bodyfat%
you say you want the toned look so I you should be looking at 2-3 sets of 8 rep for each exercise, if you can do more reps increase the weight, lifting heavy will not make you huge but it will help you maintain your existing muscle while you cut the fat and you will get stronger and improve your muscle definition. if you have the time try for 2-3 different exercises per muscle.