Keep at it, you'll get there! My first time through I could do 14/12/10 on my toes, and the remainder on my knees. I put this on the back burner for a while while I did an X rotation, and did it again this morning for the first time in 3 months - today I did the entire session on my toes.
I'd recommend trying to increase each time you do the workout, even if it's only by a single push-up. Eventually, you'll get through all 72 on your knees, then slowly, you'll get to all on your toes! You just have to really push yourself, and pay attention to proper form.
m.