In my mind, you are doing the hard part. If you are really knocking out 100 miles a week, you should get faster with any stimulus. Keep knocking out 100 miles a week if you feel good and aren't breaking down. But it doesn't have to be a 100 on the dot, mix it up based on feel. 50-60 is a big drop-off and you will likely lose fitness after awhile. If you think your quality has suffered because of the mileage, throw in a 70-80 mile week every third week.
My hunch, which one poster obviously disagreed with, is that you need to work on your speed. I don't mean speed in the 40 yard dash sense, but comfort at faster than 6:00 pace. Strides after runs (or 15 seconds pickups during a run). Once a week do some 200's at mile pace with a 200 jog. Start with 10 and slowly move towards 20. Resist the urge to go faster than mile pace, you should feel better at the end of this workout than the beginning, if done right. For the rest of the week, it really depends on the quality of your "easy" runs. If you like to crank it home some days (which likely becomes a tempo run), I wouldn't worry about V02 Max workouts for awhile. Once you start adapting to the 200's and strides (will likely take 3-4 weeks), pick one day every two weeks for reps between 800-1 miles. On the short end shoot for 5K pace, the miles closer to 8K pace. You should always be able to do another rep if you wanted to at the end of the workout.
If this is working, after about a month alternate the 200w200 jog one week with 3-4 sets of (4 * 300 w100 jog) (400 between sets). Try to run the 300's 6-8 percent slower than mile pace. This is a great 3000/5000 workout.