I feel like I am in very good shape. I can run 6 miles at around a 6:50 pace [...] run about 20 miles a week at around a 7:25 pace.
I dunno if it's the whole story or just part, but if you're running most or all of your weekly miles just :30/mi slower than your 10k race pace, you're going too fast and probably not getting the cardiovascular benefits you're looking for.
Most - even if you're an accomplished distance running training seriously - of your miles should be "easy". My 10k pace is around 6:30/mi, and my easy pace is between 8:00 and 9:00, sometimes even slower if I'm recovering from a race or hard workout.
