When it's warm I run to my workout and back. I do a casual run there, then go as fast as I can on the way back. I recommend doing this on the treadmill for your workout Hurley. Treadmill runs 1 and 3 should be relaxed, and 2 and 4 should try to beat the 1 and 3 respective times. Since you're doing 4 miles, and I'm only doing 2, I'd say don't go all out on your fast runs, just try to beat your casual runs by 20-30 seconds. It's important to push your cardio as well as the weights. With trying to beat your casual times, you'll see your average pace start to pick up.

I think a mile is a great distance to push yourself because it is longer than a sprint, but not so long that you feel like you're killing yourself, or forget what you're doing mid-run.