The first ever time I did the shred I never moved from level 1 - my core is totally shot as are my lower back muscles from 4 pregnancies in 5 years and although they say the more you work it the better it gets, for me, level 2 planking stuff is just too hard and causes a lot of pain.
In those 30 days I lost 19 pounds and so many inches I can't remember - I wrote it all down somewhere.
I have lots of Jillians DVDs now - they are all great.
It is hard and if you're just starting out, I promise the aching subsides by day 4 if you do it daily and if you are stiff/aching the best relief is to do it again.