I thought the stage 1 workouts were a bit short, too. So, I made the workouts more challenging by increasing weight every chance I could. There are a lot of alternatives you can do as well - increase height on your step-ups, do push-ups on a swiss ball, do crunches with a weight plate overhead, do a prone pike instead of a prone jackknife... I also added HIIT after each workout in stage 1.
Just because it's short doesn't mean it has to be easy.
