If you're already in great cardio shape - I wouldn't see an issue going to month two. My opinion is just do whatever keeps you interested in the program. Pure cardio is one of the easier workouts, but after week 2 you add cardio abs on top of the pure cardio workout.

Month two, as I'm sure you're aware, is 55+ minutes per workout and twice as intense. Maybe if you're looking to go at least 60 days, then mix up a little month 1 programs with month 2 to give your body a break and it will vary the workouts a little more.

I love insanity and as a gymnast and Lacrosse player I find interval training extremely helpful too! For a while I thought that insanity was just an easy quick fix to my day but just took longer to complete then I liked too. Then I really looked into it and found out that I was paying more attention to the video and schedule then I was to the overall workout! I found it very helpful to take the water breaks that they give you and even pause it sometimes to stretch it out or add a cardio twist into a day. SO insanity became an every other day thing for me and instead of doing the 60 day I did the thirty and it was so intense I just mixed up the workouts and threw a surprise, twist in there such as a run day or mixed by break days for strength days! and take the next day! At the end of the day to get your ultimate results you need to burn 3500 calories to lose that pound! So if you look at the stand still muscle workouts that you are doing then the high intensity picks up for that lean mean fighting machine physique you want!