Hi, KZN! I started doing the I-Max tape last October, and it almost murdered me the first time! It took me about 3 weeks of doing it 2X per week to get through all the intervals with no modifications; in addition to I-Max, I started doing weighted squat, lunge and hamstring work four about 10 minutes after the I-Max workout, gradually increasing those weight loads as I felt comfortable, and I think that helps. Also, because the choreography is very basic in the step interstices, I started jazzing those, substituting "Power-3's" and/or "Power-7's" in those portions that have repeaters.
I'm proud to say that, these days I am able to extend out ALL of the intervals, including the monster intervals, for 8-16 reps each, even interval #10, The Doom Interval. For me, interval #2 is the harded, half-time straight-line vertical jumps, because that's the first real extended plyometric segment and my legs really aren't on the necessary fire yet.
The only tip I have for you is: KEEP DOING IT FAITHFULLY AND CONSISTENTLY, and keep up with your progressive leg strengthening! Try to do a little bit more, either in the intervals OR in the step interstices, each time and you'd be amazed at your progress! It's a true training workout.
Good luck!
Annette