Cardio with SS?

Currently on the 2nd week of SS. Yesterday(Saturday) i decided to go to the gym and do some cardio/abs. They have a track there and it seemed like i was tired after running only a mile. I use to be able to run 2 miles no problem and even 3.

How can i incorporate cardio into SS? my overall goal is to be more fit. i have Insanity at my house. would it be ok to do the max cardio section on off days? or even on work days after my workout?