I doubt you hit a wall in endurance with 3-4 mile runs. Just slow your pace down, and run farther. If you can run 3 or 4, you can run 6. I wouldn't do the run/walk thing either, but that's just me. I would think that it would do more harm mentally, than just slowing down after 4 and biting it out. If it's Navy PRT then you don't necessarily need to run 6 miles, but I think that you should need to be able to.
Ankle weights are a horrible idea. It's not necessarily the added weight, but more the awkwardness that will lead to injuries.
BTW, what are you running on? Asphalt? Dirt? Grass? Get on some softer ground while you are upping your mileage.
I wouldn't necessarily worry about speed work right now if you can't run over 4 miles in one course. You need a larger base. You aren't in shape, yet. You'll get there, of course.
Anyway, if you up the mileage, before OCS, the most important thing you can do is listen to your body. When you are tired, rest and take it slower, when you feel good, go faster. You don't want to injure yourself before OCS, obviously.