What's the best interval running workout?

Amazing workout wanted? Exercise junkies read this please?

• Ive been packin on muscle for a few years now, and i want a hard 6 pack before the end of this summer, before my senior year. Iv'e been doing lots of cardio lately (swimming running) and still lifting weights, with lots of circuit training. Whats a HARDCORE workout that can burn a TON of calories/fat and basically make me feel like i want to die, without loosing muscle?? Don't just say only interval running and not the 300 workout.

• Answer:

  Have you heard of cross fit? Well look it up on google. Anyways here's the DL, its basically a non stop workout. Do 20 pull ups 20 push ups 20 crunches non stop for 20 minutes. You can do whatever you want though. Like somedays you can do air squats, push ups, leg lifts, then another day you can do pull ups, crunches and lunges. Just change it up, the key is don't stop for 20 minutes. If 20 minutes is to easy then up the time to 30 minutes. But crossfit is like anything else, you get out of it what you put into it. Also mix it with cardio, HIIT cardio.