It's well known among runners that interval training produces quick improvements for a few weeks, then a plateau. You are doing FOUR hard sessions per week. If you have been doing this sort of schedule for some time, you might be in that situation.
Most successful runners, right up to elite level, have a good base of lots of EASY running, and do perhaps two quality sessions per week 3/4 days apart.
The easy running / 'time on your feet' causes different kinds of adaptations to the intervals; if you can up your mileage from, say, 20 per week to 30 or 40 per week without injury, you will find your times dropping effortlessly. And this improvement can carry on for months and months, like lifting weights. But you need to run very easy to increase the mileage, then let the runs speed up later naturally when you level off.
Meanwhile, the whole idea of the hard sessions is that you do just enough to stimulate your body to adapt, then TAKE IT EASY until you are 100% recovered and actually supercompensated - thus, maybe actually at 101%. If you go out and hammer yourself the day after (e.g. your day 3) you just screw up the adaptation you might have got from day 2.
I would suggest that for the next 8 weeks you just run two hard sessions: one interval session, and one tempo run of 3+ miles at a controlled pace (use Jack Daniels tables or the McMillan calculator). Pick any one of the sessions you are now doing, or even better find a hill and do reps up and down it without a watch. Aim to finish the session feeling that you could have done a couple more reps. Repeat the session, so you can gauge if you are improving, and maybe add a rep each week instead of trying to run faster. On the other days, go for an easy run at a pace you can talk at, and lengthen the runs a bit each week.
Then with 4 weeks to go you can switch to interval training and shift up a gear again with the intensity.
Read the article "why I sucked in college" at the bottom of the LetsRun home page.
It takes real balls to stop hammering yourself and train smart, but you'll see results within a month.