I worked my way up with this workout. I think when I first did it, I could only complete the first 4 intervals.
I used to just do jumping jacks on the 5th interval blast (plie jumps).
It probably took me 6 months to finally get through the whole thing and complete the moves without any modifications.
What I found really helped me to have the endurance to complete this workout were the HIIT workouts. Once I started doing a HIIT workout once a week, I was able to finish IMAX 3. I remember being so surprised at being able to get through it without modifications, but I truly believe it was because of HIIT training.
Plus, I always keep in mind that the first 5 intervals are much more difficult than the last five. So if you get through the first 5, you'll breeze through the second half of the workout.
Just keep at it, work at it according to your own abilities, and one day you'll get through the whole thing and just be so proud of yourself!
Also, keep up diligently with your lower body strength training, because it will help you with this workout.