"INSANITY" is extended cardio, although it doesn't specifically train the muscle fibers used for running distance. Treat it as such and respect the need for the cardio systems stressed by "INSANITY" to have time to recover.
You don't get stronger while running, just like you don't get stronger while lifting weights. You get stronger while RESTING and RECOVERING from a sufficient stimulatory stressor, as your body builds adaptations to the load.
Your body can handle more work i.e. recover from more work than it can adapt to. The key is to work to what you can adapt to (which is less than what you can recover from) and to keep intensity high enough to disturb homeostasis.