Running program?

I run 3 miles 3 times a week. I want to incorporate some interval training in between these days I run. I am a former boxer, I still use a boxer's workout to stay in shape, I'm in the gym 4 to 5 days a week, work schedule permitting. Back in the day all we did was roadwork; i.e long distance running. I was wondering if any of you have an interval training program or tips I could use so I can spice up my fitness regimen. Thanks.