Every time you stop, do 15 squats and 15 situps.
This one gets tough; it's supposed to get tough! But that doesn't mean you can cheat your reps! Touch your chest to the ground each time, and lock your arms at the top. Drop to your knees if you can't do full toe pushups. Use this workout to practice your form. Every rep is valuable.
(For a lighter workout, do 75 reps. Advanced do 150 pushups.)