I love Drill Max! The first time I tried it I couldn't get through it without a few breaks & I rarely take breaks during workouts. I like you also thought I was in good shape & didn't think anything could challenge me like it did. I do this workout in some fashion (as produced, cardio/leg blast, & cardio only for a short kick butt cardio) and let me tell you it gets so much "easier" & I never take breaks during it now. That being said, like kariev mentioned your body gets efficient at adapting to the workouts you do so variety is key & the more variety I add to my collection the more I'm amazed at what I thought would be an "easier" workout can feel very advanced depending on how long it's been since I've done it and of course just how my body is feeling that day.
Anyway, stick with Drill Max & check out the premixes- This is by far my favorite workout I own & I own a lot!!! }(