I tried the Insanity "Cardio Recovery" workout last night, and completed about 26/33 minutes. I was happy with that. my feet were starting to hurt about 19 minutes in, so i took a 2 minute break and then made it through another 7 before I was done (I was getting woozy).
my legs are still a little shaky today since most of it was core/lower body based. lots of squats/lunges/and holds. I think i need to get new/better shoes. I've been wearing a pair of Reebok Easytones, which may not be the best for cardio/treadmill (feet were getting sore at the gym too).