Beginning the workouts with the decline bench sit-up not only acts as a general warm-up, but also as a wake-up for your core. Having a strong, engaged core will help you throughout the rest of the workout, especially during squats.
Five sets of 20 squats is a lot for a normal person. But, you don't want to be normal. If you did, you wouldn't be following a routine built by Arnold, who refused to live like any other man.
Make sure you're doing the cardio. Put in the miles and you'll see excellent results.