HRM Interval Training

Hello fellow BT's -

I just purchased a Polar s510 HRM monitor and I think i may be in over my head with this thing. I really need to improve my running and i bought the HRM in hopes of improving my times. i am strong in the swim and averaged 19.8 mph on my bike in my last Sprint. I'm 6'2 @ 250 lbs. While training, i usually run 3-4 miles on 9:05 or slower pace. During a race however, i can only muster 11:00 miles, i can't sustain an even pace. So here is my question.

How would you set-up my interval training? How long for warm up? How long should the intervals be? Should the intervals be based on time or on my HR? My max HR is 184. I don't know enough about training to make sure i am setting up my workouts correctly. Any assistance in this area would be greatly appreciated.

I would like to maintain the same distance for my workouts and maybe building to 5 miles per workout.

Thanks BT!