Too easy? Add free weights wherever you can (i.e., shoulder presses with squats, front raises with side lunges).
Too difficult? Take a 30 second breather after minute 5:00. Focus on form with each of the steps until you get it down perfectly. A quick Google search for any of the moves will provide photos/videos/instructions on how to best complete them.
*Disclaimer: I am not a fitness professional, and you should consult your doctor before beginning any workout program. I'm always happy to share what works for me, however!