You might have better luck posting this in the War Room, where you'll find some Marines (or former Marines, like myself) who have worked with the same problem.
This is ONLY my opinion. But when I needed to score a 275+ on the PFT, my run time was what held me back. Sprints helped me a lot. Like you said, sprint one light pole (or telephone pole) then jog/walk the next, sprint to # 3, jog/walk to #4.
I would start with 1.5 miles of that & work up to 2 miles. You don't want to kill your bod. 3 hours on a treadmill is not particularly effective for your PFT score - it's not similar enough to what you'll be doing for the PFT.
If you have extra weight, that really slows you down. Muscle weight is good - helps get you up for those pullups. Fat weight is going to drag you down like a draft horse in the mud. Streamline. Swift, silent, deadly... go for it, Marine!
How many days a week are you running now?
If you do these sprints, I'm guessing you'll be a little sore in the diaphragm area (lower ribs, mid-stomach).
Hit the track, if that helps. Sprint a quarter, jog/walk, sprint, jog/walk.
Semper Fi ~