Training and getting right is a mind field i have spent some years studying and train, got to GB level at uni (playing American football). The old saying of you are what you eat - is spot on. I train very differently, i train for power and strength but i also phase my training. At the moment i am only doing dead lifts, push press, clean, hangs, bench, standing jump. I only do 3 reps and 3 sets of these, and train them three times a week.
I am not a fan of doing sit ups every day or in fact doing them at all....If i do my exercise correctly my core will be worked to max anyway.
Anyway there is lots out there and it depends on what and where you are going with it